carp tests: test manually switch between backup and master

There's been at least one issue where we failed to correctly enter
NET_EPOCH that was triggered in this scenario.
Add a test case for it to make it easier to detect issues like this in
the future.

Reviewed by: zlei
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39115