Clarify tests for frame dimensions

test/helper.rb

@@ -199,9 +199,9 @@ def assert_file_not_exists(path)
 end
 
 def assert_frame_dimensions(range, frames)
-  frames.each_with_index do |frame, idx|
-    frame.split('x').each_with_index do |dim, idx|
-      assert range.include?(dim.to_i), "Frame #{idx}[#{idx}] should have been within #{range.inspect}, but was #{dim}"
+  frames.each_with_index do |frame, frame_index|
+    frame.split('x').each_with_index do |dimension, dimension_index |
+      assert range.include?(dimension.to_i), "Frame #{frame_index}[#{dimension_index}] should have been within #{range.inspect}, but was #{dimension}"
     end
   end
 end