Assert pytest collection succeeded in custom linter fixture

Previously the return code of pytest.main() was discarded, so a
collection error (import failure, syntax error) would silently leave
the fixture with whatever items were captured before the crash. Assert
the exit code is OK so collection errors fail the meta-tests loudly.

Author: adamtheturtle

ci/test_custom_linters.py

@@ -43,7 +43,7 @@ def all_tests() -> frozenset[str]:
     ``capsys`` would see an empty string and the test would pass vacuously.
     """
     plugin = _CollectPlugin()
-    pytest.main(
+    exit_code = pytest.main(
         args=[
             "--collect-only",
             # Disable pytest-retry to avoid:
@@ -61,6 +61,12 @@ def all_tests() -> frozenset[str]:
         ],
         plugins=[plugin],
     )
+    # Fail loudly on collection errors (import failures, syntax errors, etc.)
+    # rather than silently using whatever items were captured before the
+    # crash.
+    assert exit_code == pytest.ExitCode.OK, (
+        f"Collection failed with exit code {exit_code}."
+    )
     return frozenset(plugin.nodeids)