tests: introduce dvc.testing

This is a very early version of test utils for dvc plugins. This module
consists of existing tests with minimal changes and is going to evolve
a lot as our plugins are getting filled in and pytest-test-utils matures.

tests/conftest.py

@@ -3,6 +3,8 @@ from contextlib import suppress
 
 import pytest
 
+from dvc.testing.fixtures import *  # noqa, pylint: disable=wildcard-import
+
 from .dir_helpers import *  # noqa, pylint: disable=wildcard-import
 from .remotes import *  # noqa, pylint: disable=wildcard-import
 from .utils.scriptify import scriptify