Tests: Silence warning about missing alpha channel in render tests

Having this repeated many times makes it hard to spot actual issues.

Pull Request: https://projects.blender.org/blender/blender/pulls/118355

tests/python/modules/render_report.py

@@ -450,7 +450,11 @@ class Report:
             subprocess.check_output(command)
         except subprocess.CalledProcessError as e:
             if self.verbose:
-                print_message(e.output.decode("utf-8", 'ignore'))
+                msg = e.output.decode("utf-8", 'ignore')
+                for line in msg.splitlines():
+                    # Ignore warnings for images without alpha channel.
+                    if "--ch: Unknown channel name" not in line:
+                        print_message(line)
 
         return not failed