fix: align contrast tests with visibility diagnostics

Author: sjquant

quickthumb/_diagnostics.py

@@ -12,7 +12,6 @@
     OverlapMeasurement,
     SafeMarginPreset,
     TiledContrastMeasurement,
-    average_visible_background,
     bbox_payload,
     clear_overlap_suggestion,
     diagnostic_context,

tests/test_diagnostics.py

@@ -573,7 +573,9 @@ def test_should_use_default_text_color_for_worst_tile_contrast(self):
         from quickthumb import Canvas
 
         # given: default black text over a black background
-        canvas = Canvas(200, 120).background(color="#000000").text("default", size=36)
+        canvas = (
+            Canvas(200, 120).background(color="#000000").text("default", size=36, position=(20, 20))
+        )
 
         # when
         diagnostics = canvas.diagnose()