make unit tests run without manual opt outs file

for aaf115347e

common.py

@@ -2,7 +2,7 @@
 import base64
 from datetime import timedelta
 import logging
-import os
+from pathlib import Path
 import re
 import threading
 import urllib.parse
@@ -65,8 +65,12 @@ DOMAIN_BLOCKLIST = (
     'twitter.com',
 )
 
-with open(os.path.join(os.path.dirname(__file__), 'opt_outs.txt')) as f:
-    OPT_OUT_IDS = util.load_file_lines(f)
+opt_outs = Path(__file__).parent / 'opt_outs.txt'
+if opt_outs.exists():
+    with opt_outs.open() as f:
+        OPT_OUT_IDS = util.load_file_lines(f)
+else:
+    OPT_OUT_IDS = {'fake:bridgy-fed:test-opt-out'}
 
 # populated in models.reset_protocol_properties
 SUBDOMAIN_BASE_URL_RE = None