Found by @cmol. When rewriting the same value into the cache, we were dropping
the unchanged keys, resulting in the cache being pruned incorrectly.
Also update the tests to catch this.
Updates #12639
Change-Id: Iab67e444eb7ddc22ccc680baa2f6a741a00eb325
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>