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>

diff --git a/test/sequential/test-cluster-disconnect-leak.js b/test/parallel/test-cluster-disconnect-leak.js
similarity index 100%
rename from test/sequential/test-cluster-disconnect-leak.js
rename to test/parallel/test-cluster-disconnect-leak.js

diff --git a/test/sequential/test-cluster-disconnect-suicide-race.js b/test/parallel/test-cluster-disconnect-suicide-race.js
similarity index 100%
rename from test/sequential/test-cluster-disconnect-suicide-race.js
rename to test/parallel/test-cluster-disconnect-suicide-race.js