test: fix flaky cluster-disconnect-race

On single core Windows systems, process.send() would cause an
EPIPE because of the ordering of the IPC channel disconnect and
the process.send().

The test was originally only relevant for non-Windows platforms,
so this commit merely skips the test on Windows.

Fixes: https://github.com/nodejs/node/issues/4450
PR-URL: https://github.com/nodejs/node/pull/4457


Reviewed-By: Rich Trott <rtrott@gmail.com>