The mini cluster's admin/plugin worker keeps creating files under
admin/plugin/job_types/ for ~1s after subtests finish, while the
previous Stop() only cancelled an unobserved context and slept 500ms.
t.TempDir()'s registered RemoveAll then raced the worker and
intermittently failed with "directory not empty" (CI run 25352039081).
Manage the data dir manually so it is removed only after the mini
goroutine has exited, and wire MiniClusterCtx so cancel actually drains
master/volume/filer/admin/s3/webdav.