[binutils-gdb] Improve coverage of the PR threads/13217 regression test

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 5fd69d0ab2c42d2be0781bf3a5d60e1d5b8d05dc ***

Author: Pedro Alves <palves@redhat.com>
Branch: master
Commit: 5fd69d0ab2c42d2be0781bf3a5d60e1d5b8d05dc

Improve coverage of the PR threads/13217 regression test

- Make sure we end up with no thread selected after the detach.

- Test both "thread apply all" and "thread apply $some_threads", for
  completeness.

gdb/ChangeLog:
2017-04-13  Pedro Alves  <palves@redhat.com>

	PR threads/13217
	* gdb.threads/threadapply.exp (thr_apply_detach): New procedure.
	(top level): Call it twice, with different thread sets.