test: relax the timing window in test-child-process-fork-net2.js

In Linux, simple/test-child-process-fork-net2.js fails intermittently.

In SuSE Linux system, under network high load situations, this failure is
consistently reproducible.

The test case tests whether the TCP connections which were established between
the processes terminate in a timely and clean manner. After some iterations of
data transfer on established connections, the server is closed. The server does
not get closed immediately, instead waits for all the active connections to
terminate. A timed (200ms) callback closes the connections, which eventually
closes the server.

The start is the time when the server close is invoked.
The end is the time when the server is actually closed(onClose call back invoked).

Given that there is a minimum delay of 200ms before the connections are
terminated, expecting the elapsed time above 190 is reasonable and fair,
but looks like the leeway of 800ms for the upper bounds seem to be too
stringent, and breaking some scenarios of network load.

Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
PR-URL: https://github.com/joyent/node/pull/14129