Bug 1452212 - Use unicode literals in test_text.py

Fixes pylint:
```
tests/etl/test_text.py:34,0: Anomalous Unicode escape in byte string:
'\U'. String constant might be missing an r or u prefix. (W1402:
anomalous-unicode-escape-in-string)
```

tests/etl/test_text.py

@@ -1,4 +1,6 @@
 # -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
 from treeherder.etl.text import (astral_filter,
                                  filter_re)