tests: unset PYTEST_ADDOPTS with inner pytest runs (#537)

Ref: pytest-dev/pytest-django#412 (comment)

Author: beyondgeeks

tests/conftest.py

@@ -123,6 +123,8 @@ def create_app_file(code, filename):
     testdir.create_app_file = create_app_file
     testdir.project_root = project_root
 
+    monkeypatch.delenv('PYTEST_ADDOPTS', raising=False)
+
     return testdir