fixes for problems found while testing 3.4

[elliefm]

• clean up properly after Instance::_check_sanity() finds errors (prevents later tests from failing due to port conflicts)
• _check_sanity() does nothing if the instance's Cyrus is older than 3.5
• fixed the version check in run_mbpath() to check the correct Cyrus
• fixed some missing :min_version_3_5's

[elliefm]

Looks like a lot of "errors found in syslog" calls can also lead to a similar cascade of "port still in use" errors, so I'll also need to fix up that cleanup path too.