extproc : wait for async RPC to complete in test (#9311)

Fixes: https://github.com/grpc/grpc-go/issues/9310

This PR has the following changes: 
1. Waits for the RPC in TestExtProcChannelRetention_XDSConfigUpdate to
complete before checking interceptor close. The test was flaking because
the test was ending and the background RPC's was not complete and
context was being cancelled.
2. Pass direct errors to error handling functions instead of wrapping
them to ensure correct stream bypass or failure.

RELEASE NOTES: None