fixes: coder/internal#217

There are a couple problems:

One is that we assert the RPCs succeed, but if the pipeDialer context is canceled at the end of the test, then these assertions happen after the test is officially complete, which panics and affects other tests.

This converts these to just return the error rather than assert.

The other is that the retrier is slightly bugged: if the current retry delay is 0 AND the ctx is done, (e.g. after successfully connecting and then gracefully disconnecting), then retrier.Wait(c.ctx) is racy and could return either true or false.

Fixes the phantom redial by explicitly checking the context before dialing. Also, in the test, we assert that the controller is closed before completing the test.