gdb/testsuite: handle invalid .exp names passed in TESTS

gdb/testsuite: handle invalid .exp names passed in TESTS

I ran some tests like:

  $ make check-gdb TESTS="gdb.base/break.exp"

then, then I went to rerun the tests later, I managed to corrupt the
command line, like this:

  $ make check-gdb TESTS="gdb.base/breakff.exp"

the make command did exit with an error, but DejaGnu appeared to
report that every test passed!  The tail end of the output looks like
this:

  Illegal Argument "no-matching-tests-found"
  try "runtest --help" for option list
  		=== gdb Summary ===

  # of expected passes		115
  /tmp/build/gdb/gdb version  13.0.50.20220831-git -nw -nx -iex "set height 0" -iex "set width 0" -data-directory /tmp/build/gdb/testsuite/../data-directory

  make[3]: *** [Makefile:212: check-single] Error 1
  make[3]: Leaving directory '/tmp/build/gdb/testsuite'
  make[2]: *** [Makefile:161: check] Error 2
  make[2]: Leaving directory '/tmp/build/gdb/testsuite'
  make[1]: *** [Makefile:1916: check] Error 2
  make[1]: Leaving directory '/tmp/build/gdb'
  make: *** [Makefile:13565: check-gdb] Error 2

For a while, I didn't spot that DejaGnu had failed at all, I saw the
115 passes, and thought everything had run correctly - though I was
puzzled that make was reporting an error.

What happens is that in gdb/testsuite/Makefile, in the check-single
rule, we first run DejaGnu, then run the dg-add-core-file-count.sh
script, and finally, we use sed to extract the results from the
gdb.sum file.

In my case, with the invalid test name, DejaGnu fails, but the
following steps are still run, the final result, the 115 passes, is
then extracted from the pre-existing gdb.sum file.

If I use 'make -jN' then the 'check-parallel' rule, rather than the
'check-single' rule is used.  In this case the behaviour is slightly
different, the tail end of the output now looks like this:

  No matching tests found.

  make[4]: Leaving directory '/tmp/build/gdb/testsuite'
  find: ‘outputs’: No such file or directory
  Usage: ../../../src/gdb/testsuite/../../contrib/dg-extract-results.py [-t tool] [-l variant-list] [-L] log-or-sum-file ...

      tool           The tool (e.g. g++, libffi) for which to create a
                     new test summary file.  If not specified then output
                     is created for all tools.
      variant-list   One or more test variant names.  If the list is
                     not specified then one is constructed from all
                     variants in the files for <tool>.
      sum-file       A test summary file with the format of those
                     created by runtest from DejaGnu.
      If -L is used, merge *.log files instead of *.sum.  In this
      mode the exact order of lines may not be preserved, just different
      Running *.exp chunks should be in correct order.
  find: ‘outputs’: No such file or directory
  Usage: ../../../src/gdb/testsuite/../../contrib/dg-extract-results.py [-t tool] [-l variant-list] [-L] log-or-sum-file ...

      tool           The tool (e.g. g++, libffi) for which to create a
                     new test summary file.  If not specified then output
                     is created for all tools.
      variant-list   One or more test variant names.  If the list is
                     not specified then one is constructed from all
                     variants in the files for <tool>.
      sum-file       A test summary file with the format of those
                     created by runtest from DejaGnu.
      If -L is used, merge *.log files instead of *.sum.  In this
      mode the exact order of lines may not be preserved, just different
      Running *.exp chunks should be in correct order.
  make[3]: Leaving directory '/tmp/build/gdb/testsuite'
  make[2]: Leaving directory '/tmp/build/gdb/testsuite'
  make[1]: Leaving directory '/tmp/build/gdb'

Rather than DejaGnu failing, we now get a nice 'No matching tests
found' message, followed by some other noise.  This other noise is
first `find` failing, followed by the dg-extract-results.py script
failing.

What happens here is that, in the check-parallel rule, the outputs
directory is deleted before DejaGnu is invoked.  Then we try to run
all the tests, and finally we use find and dg-extract-results.py to
combine all the separate .sun and .log files together.  However, if
there are no tests run then the outputs/ directory is never created,
so the find command and consequently the dg-extract-results.py script,
fail.

This commit aims to fix the following issues:

 (1) For check-single and check-parallel rules, don't run any of the
 post-processing steps if DejaGnu failed to run.  This will avoid all
 the noise after the initial failure of DejaGnu,

 (2) For check-single ensure that we don't accidentally report
 previous results, this is related to the above, but is worth calling
 out as a separate point, and

 (3) For check-single, print the 'No matching tests found' message
 just like we do for a parallel test run.  This makes the parallel and
 non-parallel testing behaviour more similar, and I think is clearer
 than the current 'Illegal Argument' error message.

Points (1) and (2) will be handled by moving the post processing steps
inside an if block within the recipe.  For check-single I propose
deleting the gdb.sum and gdb.log files before running DejaGnu, this is
similar (I think) to how we delete the outputs/ directory in the
check-parallel rule.

For point (3) I plan to split the check-single rule in two, the
existing check-single will be renamed do-check-single, then a new
check-single rule will be added.  The new check-single rule can either
depend on the new do-check-single, or will ensure the 'No matching
tests found' message is printed when appropriate.