Enhance and stabilize parallel queue probe test. (#7317)
Enhance and stabilize parallel queue probe test. (#7317)

- For very slow CPUs, the requests might not actually collapse, even though we're trying hard to guarantee that. Adding an explicit sleep to the actual HTTP request should make sure that the requests actually collapse regardless.
- Added error checking for the parallel probes.
- Added a check that makes sure that the probe running after actually triggers a new request again.
knative/serving,knative/serving