assert-msg-test: Explicitly quit gdb session to avoid zombie

gdb is run in batch mode, and can leave leave the program being
executed/debugged running when the batchfile is finished. Explicitly
"quit"ing the subprocess prevents it from leaving the stray subprocess
when gdb finishes.

https://bugzilla.gnome.org/show_bug.cgi?id=731366

tests/assert-msg-test.gdb

@@ -1,3 +1,4 @@
 run
 set print elements 0
 print (char*) __glib_assert_msg
+quit