random: Exclude the timestamp from healthtest for pure sources

So-called pure sources provide entropy at regular intervals, so the
timestamp counter provides little entropy. Exclude it from health
testing for such sources.

Reviewed by: cem, emaste
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52233

(cherry picked from commit 480928ae657d81e41f8c10837cd1cf0ca87b14ae)