[gdb/testsuite] Fix gdb.threads/fork-plus-threads.exp timeout with check-run1

[gdb/testsuite] Fix gdb.threads/fork-plus-threads.exp timeout with check-run1

With gdb.threads/fork-plus-threads.exp and check-run1 we get:
...
FAIL: gdb.threads/fork-plus-threads.exp: detach-on-fork=off: \
  inferior 1 exited (timeout)
...

Fix this by calling exp_continue for new thread and thread exited messages.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-08-01  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24863
	* gdb.threads/fork-plus-threads.exp: Fix check-run1 timeout by
	calling exp_continue for new thread and thread exited messages.