[Tests] Setup needed for Connection Pool reliability testing

[cheenamalhotra]

Connection Pool reliability testing is needed for pool validation, specially with the new incoming connection pool design work, led by @mdaigle #3356.

This test framework should cover connection pool behaviors under stress, high CPU load and concurrent runs. Some of the initial work came through PR #3632 (ConnectionPoolStressTest) but is incomplete in its current state. It needs a new home, reworked and redesigned to do more elaborate testing.