skip tests whose optional dependency is not installed

With pywikidiff2 and mediawiki-utilities no longer installed by default,
the tests covering --diff, -p wikidiff2, and -p legacy cannot run on a
base install. Mark them so a base install reports skips rather than
failures, and keep the markers in wikiq_test_utils.py so all three test
modules share one definition of what each feature needs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
(cherry picked from commit 59d5a9d46a04320bad5d81e0edf5ecd11f9c2a9b)

test/wikiq_test_utils.py

@@ -1,8 +1,27 @@
+import importlib.util
 import os
 import shutil
 import subprocess
 from typing import Final, Union
 
+import pytest
+
+
+def _installed(module: str) -> bool:
+    return importlib.util.find_spec(module) is not None
+
+
+# wikiq's two optional dependencies. Tests exercising --diff, -p wikidiff2, or
+# -p legacy cannot run on a base install, so they skip rather than fail.
+requires_pywikidiff2 = pytest.mark.skipif(
+    not _installed("pywikidiff2"),
+    reason="needs the optional pywikidiff2 extension (--diff, -p wikidiff2)",
+)
+requires_mediawiki_utilities = pytest.mark.skipif(
+    not _installed("mw"),
+    reason="needs the optional mediawiki-utilities package (-p legacy)",
+)
+
 TEST_DIR: Final[str] = os.path.dirname(os.path.realpath(__file__))
 WIKIQ: Final[str] = os.path.join(os.path.join(TEST_DIR, ".."), "src/wikiq/__init__.py")
 TEST_OUTPUT_DIR: Final[str] = os.path.join(TEST_DIR, "test_output")