tor-circmg: Make `GuardMgr` mandatory

I think this covers #1465 (closed).

Functions that took Option<&GuardMgr> now take only &GuardMgr. The tests are updated to use a guardmgr instead of None, and three tests were removed that tested the behaviour when not using a guardmgr.

Also adjusted the path::exitpath::test::by_ports test which became flaky when using a guardmgr. select_guard, when given a chosen exit relay, only ensures that the guard and chosen exit are not in the same family. It does not ensure that the guard and exit do not share an extended family. Since this "chosen exit" functionality isn't actually being used anywhere and I'm not sure what it's original intended purpose was, I left the existing path selection behaviour and just relaxed the test slightly. Let me know if I should do something else here instead.

Please let me know what changes to make, even if they're small style things :)