tmpfs: Improve file time tests.

• The empty test case no longer fails because 89f1dcb3eb46 causes empty files to bypass the bug.
• The bug still exists, so add a test case which exercises it.
• While here, tighten up some of the checks.

A similar patch has been submitted upstream.

PR: 274615
X-MFC-With: 89f1dcb3eb46
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44609