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

submitting on behalf of gireeshpunathil as he is out on vacation this week. See discussion in https://github.com/nodejs/node/issues/3245 in respect to detailed investigation of test behaviour