test: convert pytest-style tests to dir_helpers fixtures

[Suor]

This is a continuation of #2837.

This might be done on a per module basis to get a taste of it, leave a comment here if you are migrating particular test module to prevent double effort. I will convert whatever is not done by the end of the sprint.

So @MrOutis, @pared, @efiop, this goes after loose ends are closed as we agreed on planning. Ping me here if you have issues with any new fixtures.

When this is done, someone should kill chimera pytest fixtures in conftest.py: repo_dir, git, dvc_repo, erepo, git_erepo. temporary_windows_drive should be inlined or moved closer to its usage.

• migrate old pytest-style tests
• kill chimera fixtures
• inline or move temporary_windows_drive, stop using repo_dir there

[pared]

migrating test_add

migrating test_analytics

Advice extracted from https://github.com/iterative/dvc/pull/2898/files#r354523910

The order doesn't matter to python, but it matters to reader. Let's list fixtures in the same order everywhere: tmp_dir, scm (if used), dvc. This is chronological: temp dir is created, then git init, then dvc init.

[Suor]

Migrating test_ignore.

[pared]

Migrating test_move