tests: eager config for pytest-cov (pytest-dev#533)

Via pytest-dev#412 (comment).

Author: blueyed

tox.ini

@@ -28,7 +28,12 @@ deps =
 
     postgres: psycopg2
 setenv =
-     PYTHONPATH = {toxinidir}:{env:PYTHONPATH:}
+    PYTHONPATH = {toxinidir}:{env:PYTHONPATH:}
+
+    # Ref: https://github.com/pytest-dev/pytest-django/pull/412#issuecomment-340077539
+    COV_CORE_SOURCE={toxinidir}
+    COV_CORE_CONFIG={toxinidir}/.coveragerc
+    COV_CORE_DATAFILE={toxinidir}/.coverage.eager
 
 passenv = PYTEST_ADDOPTS
 usedevelop = True