Make gdb.base/a2-run.exp use $inferior_spawn_id and gdb_test_stdio

Make gdb.base/a2-run.exp use $inferior_spawn_id and gdb_test_stdio

This one is a little more complicated than the other patches in this
series, because of the exit status wrapper handling, requiring a
little state machine.

gdb/testsuite/ChangeLog:
2015-07-29  Pedro Alves  <palves@redhat.com>

	* gdb.base/a2-run.exp (saw_usage, saw_exit_wrapper)
	(saw_spurious_output): Expect inferior output from
	$inferior_spawn_id.  Use gdb_test_stdio.