tests: Skip buffer count test if wl_drm isn't present

This skips the test when running on the headless backend.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
(Presumably) Reviewed-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/tests/weston-test-client-helper.c b/tests/weston-test-client-helper.c
index e21439c..518e899 100644
--- a/tests/weston-test-client-helper.c
+++ b/tests/weston-test-client-helper.c
@@ -482,6 +482,8 @@
 					 &wl_test_interface, 1);
 		wl_test_add_listener(test->wl_test, &test_listener, test);
 		client->test = test;
+	} else if (strcmp(interface, "wl_drm") == 0) {
+		client->has_wl_drm = true;
 	}
 }