test: remove unused empty fixture

The not-quite-empty directory called "empty" in fixtures was replaced in
tests with `tmpdir.path` (after calling `tmpdir.refresh()` to ensure
that it is empty). The fixture is removed in this commit.

PR-URL: #21044
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>