Subject: [PATCH] Compare font content in the overlap-removal regression

PFA_SKIP ends with a separator, producing an empty prefix that matches every
line. Remove the empty prefix for this test so glyph differences are checked,
and assert that tx exits successfully before inspecting the output.

--- a/tests/tx_test.py
+++ b/tests/tx_test.py
@@ -1064,6 +1064,8 @@
     output_path = get_temp_file_path()
     args = [TOOL, '-t1', '+V', '-o', output_path, input_path]
-    subprocess.call(args)
-    assert differ([expected_path, output_path, '-s', PFA_SKIP[0]])
+    subprocess.check_call(args)
+    # An empty skip prefix matches every line and hides font differences.
+    skip_headers = SPLIT_MARKER.join(filter(None, PFA_SKIP[0].split(SPLIT_MARKER)))
+    assert differ([expected_path, output_path, '-s', skip_headers])