test: harden test-child-process-fork-regr-gh-2847

test-child-process-fork-regr-gh-2847 could fail depending
on timing and how messages were packed into tcp packets.
If all of the requests fit into one packet then the test
worked otherwise, otherwise errors could occur.  This PR
modifies the test to be tolerant while still validating that
some of the connection can be made succesfully

Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3459