Add a ClientConn level test for idleness race

[easwars]

See #6538 which was never caught by our existing tests.

One option for this test is to run cc.exitIdleMode and cc.enterIdleMode concurrently and ensure that things work as expected.

[Aditya-Sood]

hi @easwars, can I pick this up?

[dfawley]

This should wait until #6566 is merged.

[dfawley]

Let's also clean up enterIdleMode: defer the unlock and also do the channelz event synchronously instead of in the goroutine at the end.