tests: Cleanup superfluous headers

Mostly remove headers that aren't actually needed for anything.
Add stdint.h to permit dropping xf86drm.h, which is otherwise unneeded.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Acked-by: Marek Chalupa <mchqwerty@gmail.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/tests/button-test.c b/tests/button-test.c
index d5ad813..a784258 100644
--- a/tests/button-test.c
+++ b/tests/button-test.c
@@ -23,6 +23,7 @@
 #include "config.h"
 
 #include <linux/input.h>
+
 #include "weston-test-client-helper.h"
 
 TEST(simple_button_test)