gdb/testsuite: avoid temporary file in gdb/testsuite (unittest.exp)

gdb/testsuite: avoid temporary file in gdb/testsuite (unittest.exp)

I spotted that the gdb.gdb/unittest.exp script causes a temporary file
inserters_extractors-2.txt to be created in build/gdb/testsuite/
instead of in build/gdb/testsuite/output/gdb.gdb/unittest/.

This is because some of the 'maint selftest' tests create temporary
files in GDB's current directory, specifically, the two source files:

  gdb/unittests/basic_string_view/inserters/wchar_t/2.cc
  gdb/unittests/basic_string_view/inserters/char/2.cc

both create a temporary file called inserters_extractors-2.txt, though
we only run the second of these as part of GDB's selftests.

I initially proposed just using GDB's 'cd' command in unittest.exp to
switch to the test output directory before running the selftests,
however, Pedro pointed out that there was a risk here that, if GDB
crashed during shutdown, the generated core file would be left in the
test output directory rather than in the testsuite directory.  As a
result, our clever core file spotting logic would fail to spot the
core file and alert the user.

Instead, I propose this slightly more involved solution.  I've added a
new with_gdb_cwd directory proc, used like this:

  with_gdb_cwd $directory {
    # Tests here...
  }

The new proc temporarily switches to $directory and then runs the
tests within the block.  After running the tests the previous current
working directory is restored.

Additionally, after switching back to the previous cwd, we check that
GDB is still responsive.  This means that if GDB crashed immediately
prior to restoring the previous directory, and left the core file in
the wrong place, then the responsiveness check will fail, and a FAIL
will be emitted, this should be enough to alert the user that
something has gone wrong.

With this commit in place the unittest.exp script now leaves its
temporary file in the test output directory.