custom comparison for SpecialPhrase

Duplicate elemination only works when a custom hash/equal function
is implemented that is based on the members.

test/python/tools/test_import_special_phrases.py

@@ -17,11 +17,6 @@ from nominatim.errors import UsageError
 
 from cursor import CursorForTesting
 
-@pytest.fixture
-def testfile_dir(src_dir):
-    return src_dir / 'test' / 'testfiles'
-
-
 @pytest.fixture
 def sp_importer(temp_db_conn, def_config, monkeypatch):
     """