Add coverage to runtests. Closes #2936.

Author: tomchristie

Diff for: requirements/requirements-testing.txt

@@ -1,3 +1,4 @@
 # PyTest for running the tests.
 pytest==2.6.4
 pytest-django==2.8.0
+pytest-cov==1.8.1

Diff for: runtests.py

@@ -8,7 +8,7 @@
 import pytest
 
 PYTEST_ARGS = {
-    'default': ['tests', '--tb=short', '-s'],
+    'default': ['tests', '--tb=short', '-s', '--cov', 'rest_framework'],
     'fast': ['tests', '--tb=short', '-q', '-s'],
 }