node tests: Remove crufty fakes/mocks.

[showell]

It's easy to run the node coverage reports and track down code in the tests themselves that never gets called. Usually these are fakes/mocks that aren't really needed any more to test the code.

We should probably come up with a scheme to lock down these files, too, although there are some nuisances, like it's often legit to have a little bit of dead code in the node tests to help troubleshoot things like switch reaching default.

[zulipbot]

Hello @zulip/server-testing members, this issue was labeled with the "area: testing-coverage" label, so you may want to check it out!