Add a test for the initial channel state while waiting for the first name resolver update

[dfawley]

I don't think this is currently tested, and I'm not clear whether we have a bug. We should have a test that does the following:

1. Create a channel with a manual name resolver set to not produce any updates when constructed.
2. Confirm the channel's State() == Idle.
3. Two forks for this test:
   1. Call ClientConn.Connect() and confirm the channel's State() == Connecting immediately upon returning. (It's possible this is racy but it should not be.)
   2. Perform an RPC, which will block since the name resolver never produced anything. In a separate goroutine, confirm the state eventually changes to Connecting using testutils.AwaitState.
4. Call internal.EnterIdleModeForTesting and then perform (2) and (3) above.

[janardhanvissa]

@arjan-bal Please assign me this issue.