Fix test_gdb_complete_tab_multiple race

Fix test_gdb_complete_tab_multiple race

Running 'make check-read1 TESTS="gdb.base/options.exp"' revealed a
race in test_gdb_complete_tab_multiple.  There's a gdb_test_multiple
call that expects a prompt in the middle of the regexp.  That's racy
because gdb_test_multiple includes a built-in FAIL pattern for the
prompt, which may match if gdb is slow enough to produce the rest of
the output after the prompt.

Fix this in the usual way of splitting the matching in two.

gdb/testsuite/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	* lib/completion-support.exp (test_gdb_complete_tab_multiple):
	Split one gdb_test_multiple call in two to avoid a race.