Include coverage in the list of requirements for the tests

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

tests_requirements.txt

@@ -1,6 +1,7 @@
 bcrypt
 beautifulsoup4
 black; python_version >= '3.6' # Only available on py3.6+
+coverage
 cryptography
 eventlet
 fedmsg