test: move cluster tests to parallel

Two cluster tests have recently changed so that they are no longer
resource intensive. Move them back to parallel.

Ref: https://github.com/nodejs/node/pull/4736
Ref: https://github.com/nodejs/node/pull/4739
PR-URL: https://github.com/nodejs/node/pull/4774


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>