Fixed scanner_test which was broken.

The monkeypatching made to hscommon.io wasn't correctly transferred to Path
after the migration at [dfc82cd].

core/tests/scanner_test.py

@@ -38,6 +38,7 @@ def pytest_funcarg__fake_fileexists(request):
     # for file existence before doing the match grouping.
     monkeypatch = request.getfuncargvalue('monkeypatch')
     monkeypatch.setattr(io, 'exists', lambda _: True)
+    monkeypatch.setattr(Path, 'exists', lambda _: True)
 
 def test_empty(fake_fileexists):
     s = Scanner()