tests: improve test_import_dir

[dtrifiro]

In some cases, such as with dvc-gs, test_import_dir cannot succeed because the stage/dir md5 cannot be known in advance, see https://github.com/iterative/dvc-gs/issues/7#issuecomment-1218497067 for more context

[dtrifiro]

This will probably will have to be tested against dvc plugins

[efiop]

In some cases, such as with dvc-gs, test_import_dir cannot succeed because the stage/dir md5 cannot be known in advance

With import discussion in mind, let's pause this PR for a moment and get back to it in a few weeks. If we merge we'll need to adjust the tests anyway, so I want to make sure that the import changes won't mitigate this.

[dtrifiro]

Waiting for new tree before re-assessing this