Fix 'gdb.base/quit.exp hangs forever' if the test fails

Fix 'gdb.base/quit.exp hangs forever' if the test fails

The [wait -i $gdb_spawn_id] in the test is dangerous in the sense that
it won't be subject to timeout logic.  So if GDB fails quiting, this
testcase hangs forever, hanging the test run with it.  See:
  https://sourceware.org/ml/gdb-patches/2016-10/msg00728.html

Instead of 'wait'ing directly, use gdb_test_multiple and expect 'eof'.

Tested that the testcase no longer hangs by hacking the test to send
"info threads" instead of "quit".

Tested with
  --target_board={unix, native-gdbserver,native-extended-gdbserver}
and tested with
  --host_board=local-remote-host
as well.

gdb/testsuite/ChangeLog:
2017-10-20  Pedro Alves  <palves@redhat.com>

	* gdb.base/quit.exp: Use gdb_test_multiple and expect 'eof' before
	'wait -i'.  Use gdb_assert and remote_close.