ringhash: more e2e tests from c-core

[atollena]

Follow up to https://github.com/grpc/grpc-go/pull/7271 to fix https://github.com/grpc/grpc-go/issues/6072.

This adds a dozen more end to end tests.

There are tests that I did not port, specifically:

• TestRingHash_SwitchToLowerPriorityAndThenBack was also flaky when ported as-is, I also removed it while investigating.

• TestRingHash_TransientFailureSkipToAvailableReady was flaky when ported as-is, so I removed it while investigating.

• TestRingHash_ReattemptWhenAllEndpointsUnreachable was flaky when ported as-is, so I removed it while investigating.

• TestRingHash_ContinuesConnectingWithoutPicksOneSubchannelAtATime, I'm not sure we implement this behavior, and if we do, it's not working the same way as in c-core, where the order of subchannel connection attempts is based on the resolver address order rather than the ring order.

I will follow up with fixes for each one of the remaining tests.

RELEASE NOTES: none