test: move test-child-process-fork-getconnections to parallel

Now that the test uses an open port assigned by the operating system
rather than a hardcoded common.PORT, it can be moved to parallel.

PR-URL: #30749
Reviewed-By: Denys Otrishko <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>

Author: Trott