Potential bug when testing run with metrics_no_cache

[pared]

In our test base, TestDirFixture.CODE copies src to target. It can cause problem if one decides to write a test like:

dvc_repo.add(repo_dir.FOO)
metric_file = "metric.txt"
dvc_repo.run(
        deps=[repo_dir.FOO],
        metrics_no_cache=[metric_file],
        cmd="python {} {} {}".format(repo_dir.CODE, repo_dir.FOO, metric_file),
    )

In this case metric is not supposed to be in cache. But due to fact that it is identical as repo_dir.FOO it is inside cache, so we might end up "finding" it for example here. Which can lead to some unexpected behaviour.

Currently, there is no test using CODE to create metric, so I would not prioritize this task, though we should do something to discourage or prevent this use case.

[efiop]

Closing as stale