time: uncomment t.Fatalf call in a test

The t.Fatalf call was commented out since it was added in CL 568341.
We can either remove the check or enable the call. Since the test is
checking anyway (and is passing) we might as well enable the t.Fatalf.

Change-Id: Iaed15f9c04cdd6d489209f50b7bb46a15c1eec67
Reviewed-on: https://go-review.googlesource.com/c/go/+/781681
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>