Cherry-pick fixing of connection priority tests (#4739)

* added unique pointer for connection (#4718)

* Fix priority operation flaky test (#4615)

* Sleep longer to flush queue for sure

* cleanup environment for each test case

* sleep

* GetParam instead of sleep

* GetParam 50 times

* experimental, handshake just before enqueueing stream operations. Locally works for 1000 times

* connection start once all operations are queued

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Co-authored-by: Gaurav Singh <[email protected]>