[gdb/testsuite] Fix target_supports_scheduler_locking raciness

[gdb/testsuite] Fix target_supports_scheduler_locking raciness

When calling gdb_start_cmd, it's the caller's responsibility to wait for gdb
to return to the prompt.  In target_supports_scheduler_locking, that's not the
case, and consequently, target_supports_scheduler_locking fails spuriously.

Fix by using runto_main instead.

Build and reg-tested on x86_64-linux.

2018-10-09  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (target_supports_scheduler_locking): Replace gdb_start_cmd
	with runto_main.