Improve core file path detection & put cores in output dir

Improve core file path detection & put cores in output dir

After a testrun, I noticed that I have some kernel-produced cores for
testcase programs, under build/gdb/testsuite/, which shouldn't be
there:

 $ ls -1 testsuite/core.*
 testsuite/core.annota1.1274351.nelson.1656004407
 testsuite/core.annota3.1288474.nelson.1656004414
 testsuite/core.exitsignal.1240674.nelson.1656004391

I have my core pattern setup like this:

 $ cat /proc/sys/kernel/core_pattern
 core.%e.%p.%h.%t

That's:

 %e: executable filename
 %p: pid
 %h: hostname
 %t: UNIX time of dump

so it's easy to tell which program produced the core from the core
file name.

From above, we can tell that the corresponding testcases are
gdb.base/annota1.exp, gdb.base/annota3.exp and
gdb.base/exitsignal.exp.

At least gdb.base/annota1.exp and gdb.base/annota3.exp have code in
them to delete the core file.  However, that isn't working for me,
because said code only looks for cores named exactly either "core" or
"core.PID", and my core_pattern doesn't match that.

Another issue I noticed, is that I have not been running
gdb.base/bigcore.exp, for a similar reason.  I get:

  Program terminated with signal SIGABRT, Aborted.
  The program no longer exists.
  (gdb) PASS: gdb.base/bigcore.exp: signal SIGABRT
  UNTESTED: gdb.base/bigcore.exp: can't generate a core file

But I actually have a core file under the testcase's output dir:

 $ find . -name "core.*"
 ./testsuite/outputs/gdb.base/bigcore/core.bigcore.2306705.nelson.1656005213
 $

This commit fixes these things, by adding a find_core_file routine
that searches core files in a way that works with my core pattern as
well.  This then also adds a convenience remove_core routine as a
wrapper around find_core_file that removes the found core file.

In addition, it changes some testcases that expect to have their
program dump core, to switch the inferior's cwd to the testcase's
output dir, so that the core is dumped there instead of in
build/gdb/testsuite/.  Some testcases were already doing that, but not
all.  The idea is that any core file dumped in build/gdb/testsuite/ is
an unexpected core file.  The next patch will add a count of such
unexpected core files to gdb.sum.

Another change is that the directory changing is now done with "set
cwd" instead of with "cd".  "set cwd" only affects the inferior cwd,
while "cd" affects GDB's cwd too.  By using "set cwd" instead of "cd",
if GDB dumps core in these testcases, the GDB core dump will still end
up in build/gdb/testsuite/, and can thus be detected as an unexpected
core.

Change-Id: I45068f21ffd4814350aaa8a3cc65cad5e3107607