skip test_infobox when its fixture files are absent

test_infobox reads test/test_diff_revisions/test_infobox_from and
test_infobox_to, which were never committed to the repository, so the
test always failed with FileNotFoundError. Skip it when the fixtures
are missing so it runs again if they are ever added.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/test_wiki_diff_matcher.py

@@ -1,5 +1,6 @@
 from itertools import chain
 from functools import partial
+import os
 import re
 import pytest
 import pytest_asyncio
@@ -278,6 +279,10 @@ def test_paragraph_move_and_change():
     assert_equal_enough(a, rev1)
     assert_equal_enough(b, rev2)
 
+@pytest.mark.skipif(
+    not os.path.exists("test/test_diff_revisions/test_infobox_from"),
+    reason="test_infobox_from/_to fixture files are not in the repository",
+)
 def test_infobox():
     rev1 = open("test/test_diff_revisions/test_infobox_from").read()
     rev2 = open("test/test_diff_revisions/test_infobox_to").read()