Collect tests from __init__.py files if they match 'python_files'

Fix #3773

testing/example_scripts/collect/collect_init_tests/pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+python_files = *.py

testing/example_scripts/collect/collect_init_tests/tests/__init__.py

@@ -0,0 +1,2 @@
+def test_init():
+    pass

testing/example_scripts/collect/collect_init_tests/tests/test_foo.py

@@ -0,0 +1,2 @@
+def test_foo():
+    pass