test: add description to snapshot test

Author: TomJGooding

tests/snapshot_tests/test_snapshots.py

@@ -3723,7 +3723,10 @@ def on_mount(self) -> None:
 
 
 def test_panel_border_title_colors(snap_compare):
-    """Regression test for https://github.com/Textualize/textual/issues/5548"""
+    """Regression test for https://github.com/Textualize/textual/issues/5548
+
+    You should see four labels with panel type borders. The border title colors
+    should match the description in the label."""
 
     class BorderTitleApp(App):
         CSS = """