Passing in an explicit testing.TB gives two benefits:
1. It allows the helper to fail the test itself, instead of returning
an error to the caller. A non-nil error invariably fails the
calling test, and none of these callers bother to add detail to the
error when logging it anyway so returning the error just added
noise to the test bodies.
2. It allows the helper to use t.Cleanup to perform any needed cleanup
tasks, which will be used in CL 370695 to clean up temp directories
used as namespaces for unix socket paths.