test: remove the requirement for a Layout file (#661)

Previously we required a layout file for any tablet with pad
buttons. This somewhat prevents us from adding tablet files for unknown
tablets where we can get most of the information from external sources,
e.g. see #659. As a result we're missing support for these tablets
altogether when we could have at least partial support.

Let's drop this requirement in our tests, users who want their tablet
working properly can submit layout files.

test/test-tablet-svg-validity.c

@@ -233,19 +233,6 @@ struct fixture {
 	xmlNodePtr root;
 };
 
-static void
-test_filename(struct fixture *f, gconstpointer data)
-{
-    const WacomDevice *device = data;
-    const char *filename;
-
-    filename = libwacom_get_layout_filename(device);
-    if (libwacom_get_num_buttons(device) > 0) {
-	    g_assert_nonnull(filename);
-	    g_assert_cmpstr(filename, !=, "");
-    }
-}
-
 static void
 test_svg(struct fixture *f, gconstpointer data)
 {
@@ -368,7 +355,6 @@ static void setup_tests(WacomDevice *device)
 	if (g_str_equal(name, "Generic"))
 		return;
 
-	add_test(device, test_filename);
 	if (!libwacom_get_layout_filename(device))
 		return;