tests: Sprinkle missing igt_exit() where needed.

Yay for breaking piles of tests.

This regression has been introduced with

commit 5782eca1e1
Author: Tim Gore <tim.gore@intel.com>
Date:   Wed Oct 1 13:25:20 2014 +0100

    lib/igt_core.c: disable lowmemorykiller during tests

Cc: Tim Gore <tim.gore@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

tests/gem_ctx_basic.c

@@ -166,5 +166,5 @@ int main(int argc, char *argv[])
 	free(threads);
 	close(fd);
 
-	return 0;
+	igt_exit();
 }