tests.7: Remove an unused configuration variable

No existing tests require it, and I cannot understand what kinds of test
scenarios are supposed to require it. Just remove it.

While here, improve the documentation of test variables a bit.

Reviewed by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56604

(cherry picked from commit 6bd97b5f3778aa36bcf89ff870bb1483b301a9be)