extproc: fix TestObservabilityMidStreamFailDeny race (#9295)

Fixes: https://github.com/grpc/grpc-go/issues/9289

This PR changes `TestObservabilityMidStreamFailDeny` to send and recv
message until it gets an error. The test was failing because send and
recv call directly in dataplane stream without waiting for error from
proc stream. Error from proc stream is received async in
`recvAndDiscardResponses` loop. SO it can happen that 10 send and recv
happen very fast before the proc server send error frame.

RELEASE NOTES: None

#ext-proc-a93