Fix unstable test names in gdb.threads/attach-into-signal.exp

Fix unstable test names in gdb.threads/attach-into-signal.exp

Currently, if you diff testsuite/gdb.sum of two testsuite runs you'll
often see spurious hunks like these:

  -PASS: gdb.threads/attach-into-signal.exp: nonthreaded: attempt 2: attach (pass 2), pending signal catch
  +PASS: gdb.threads/attach-into-signal.exp: nonthreaded: attempt 1: attach (pass 2), pending signal catch
   PASS: gdb.threads/attach-into-signal.exp: successfully compiled posix threads test case
   PASS: gdb.threads/attach-into-signal.exp: threaded: handle SIGALRM stop print pass
  -PASS: gdb.threads/attach-into-signal.exp: threaded: attempt 1: attach (pass 1), pending signal catch
  -PASS: gdb.threads/attach-into-signal.exp: threaded: attempt 1: attach (pass 2), pending signal catch
  +PASS: gdb.threads/attach-into-signal.exp: threaded: attempt 2: attach (pass 1), pending signal catch
  +PASS: gdb.threads/attach-into-signal.exp: threaded: attempt 4: attach (pass 2), pending signal catch

Fix this by removing the "attempt $attempt" test prefix.  The attempt
number can be retrieved from gdb.log instead, since the testcase is
already using "verbose -log" to that effect.

(The 'with_test_prefix "stoppedtry $stoppedtry"' prefix is unnecessary
too, because inside that block there are no pass/fail calls.  In fact
the block includes a comment saying:

  # No PASS message as we may be looping in multiple
  # attempts.

but I'll drop that whole loop in the next patch instead.)

After this commit we'll show:

  PASS: gdb.threads/attach-into-signal.exp: nonthreaded: handle SIGALRM stop print pass
  PASS: gdb.threads/attach-into-signal.exp: nonthreaded: attach (pass 1), pending signal catch
  PASS: gdb.threads/attach-into-signal.exp: nonthreaded: attach (pass 2), pending signal catch
  PASS: gdb.threads/attach-into-signal.exp: successfully compiled posix threads test case
  PASS: gdb.threads/attach-into-signal.exp: threaded: handle SIGALRM stop print pass
  PASS: gdb.threads/attach-into-signal.exp: threaded: attach (pass 1), pending signal catch
  PASS: gdb.threads/attach-into-signal.exp: threaded: attach (pass 2), pending signal catch

(I've avoided reindenting to make the patch easier to maintain/read.
I'll reindent the blocks after this is in.)

gdb/testsuite/ChangeLog:
2017-10-24  Pedro Alves  <palves@redhat.com>

	* gdb.threads/attach-into-signal.exp (corefunc): Remove "attach
	$attempt" test prefix.