tests: do not save PNG on success

Do not write out PNG files for successful screenshot tests. It clutters
the build directory, but the biggest reason is to keep the CI artifacts
smaller.

internal-screenshot test still writes a PNG, it's good to keep one PNG
written so that we exercise the PNG writing code always.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

tests/weston-test-client-helper.c

@@ -1673,10 +1673,11 @@ write_visual_diff(pixman_image_t *ref_image,
 /**
  * Take a screenshot and verify its contents
  *
- * Takes a screenshot and writes the image into a PNG file named with
+ * Takes a screenshot and compares the contents to the given reference
- * get_test_name() and seq_no. Compares the contents to the given reference
  * image over the given clip rectangle, reports whether they match to the
- * test log, and if they do not match writes a visual diff into a PNG file.
+ * test log, and if they do not match writes a visual diff into a PNG file
+ * and the screenshot into another PNG file named with get_test_name() and
+ * seq_no.
  *
  * The compositor output size and the reference image size must both contain
  * the clip rectangle.
@@ -1711,12 +1712,11 @@ verify_screen_content(struct client *client,
 	pixman_image_t *ref = NULL;
 	char *ref_fname = NULL;
 	char *shot_fname;
-	bool match;
+	bool match = false;
 
 	shot = capture_screenshot_of_output(client);
 	assert(shot);
 	shot_fname = screenshot_output_filename(test_name, seq_no);
-	write_image_as_png(shot->image, shot_fname);
 
 	if (ref_image) {
 		ref_fname = screenshot_reference_filename(ref_image, ref_seq_no);
@@ -1736,9 +1736,11 @@ verify_screen_content(struct client *client,
 		pixman_image_unref(ref);
 	} else {
 		testlog("No reference image, shot %s: FAIL\n", shot_fname);
-		match = false;
 	}
 
+	if (!match)
+		write_image_as_png(shot->image, shot_fname);
+
 	free(ref_fname);
 	buffer_destroy(shot);
 	free(shot_fname);