Cleanup: Better logging for imbuf tests

Recent failures requiring investigation have exposed some shortcomings
that this addresses:
- When creating the diff image for offline comparison, use a higher
  threshold to prevent idiff from printing more output which will often
  contradict the primary failure output just above it (very confusing)
- For metadata failures, make sure these get printed so it's obvious
  what kind of failure we're dealing with

Pull Request: https://projects.blender.org/blender/blender/pulls/107058

tests/python/bl_imbuf_load.py

@@ -32,6 +32,7 @@ class ImBufTest(AbstractImBufTest):
         colorspace = img.colorspace_settings.name
         alpha_mode = img.alpha_mode
         actual_metadata = f"{channels=} {is_float=} {colorspace=} {alpha_mode=}"
+        expected_metadata = ""
 
         # Save actual metadata
         out_metadata_path.write_text(actual_metadata, encoding="utf-8")
@@ -52,10 +53,14 @@ class ImBufTest(AbstractImBufTest):
 
             failed = True
 
-        if failed and self.update:
-            # Update reference if requested.
-            ref_metadata_path.write_text(actual_metadata, encoding="utf-8")
-            failed = False
+        if failed:
+            if self.update:
+                # Update reference if requested.
+                ref_metadata_path.write_text(actual_metadata, encoding="utf-8")
+                failed = False
+            else:
+                print_message(
+                    "Expected [{}] but got [{}]".format(expected_metadata, actual_metadata))
 
         return not failed
 

tests/python/modules/imbuf_test.py

@@ -73,12 +73,14 @@ class AbstractImBufTest(unittest.TestCase):
             shutil.copy(out_filepath, ref_filepath)
             failed = False
 
-        # Generate diff image.
+        # Generate diff image (set fail thresholds high to reduce output spam).
         diff_img = str(self.diff_dir.joinpath(out_name + ".diff.png"))
         command = (
             str(self.idiff),
-            "-o", diff_img,
+            "-fail", "1",
+            "-failpercent", "100",
             "-abs", "-scale", "16",
+            "-o", diff_img,
             ref_filepath,
             out_filepath
         )