clusterimpl: fix test logic waiting for calls to ResolveNow (#9071) Addresses one test in https://github.com/grpc/grpc-go/issues/8995 This is the sequence of events happening in the test: - gRPC channel moves to READY and is able to make an RPC. - The server goes down, causing the client transport to close. This causes the gRPC channel to invoke `ResolveNow`. See: https://github.com/grpc/grpc-go/blob/06fc26a196350499dd0cf2dabdd5cf11bbee01a7/clientconn.go#L1517 - Subchannel moves to IDLE. - `pick_first` moves channel state to IDLE. `round_robin` reacts with a call to `ExitIdle`, causing `pick_first` to reconnect. - Reconnection attempt fails as the server is still down, and the subchannel moves to TRANSIENT_FAILURE. - This causes the `clusterimpl` policy to call `ResolveNow` There are two calls to `ResolveNow`, but the test was only expecting one, and therefore the second one was blocking until the test context expires. RELEASE NOTES: none
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