By splitting dvc.cli into a module, we'll be able to share cli-related
utilities, split parser-related stuffs into smaller parts, improve
reusability and improve API for command extensions in the future.
With introduction of https://github.com/iterative/dvc-plugins,
all important tests are being ran there, so there is no need
for dvc repo to run them again.
This PR also removes some redundant/broken remote tests.
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.