test: use the connectivity state watcher API (#8708)

test: use the connectivity state watcher API (#8708)

The current tests are using an LB policy to record the state transitions
of subchannels. But these tests are meant to test the connectivity state
transitions of the channel.

Also, in a follow-up PR I will be making the change to transition the
channel to CONNECTING as soon as it exits IDLE
(https://github.com/grpc/grpc-go/issues/7686). With that change, the
current test don't work anymore. So, we will have to change these tests
anyway.

So, I took the opportunity to clean up them and use the connectivity
state watcher API to record the state transitions of the grpc channel
and compare them against the expected states.

RELEASE NOTES: none