test: move resource intensive tests to sequential

To enable greater parallelization of tests on CI, move resource
intensive tests out of parallel and into sequential.

Ref: https://github.com/nodejs/node/pull/4476
PR-URL: https://github.com/nodejs/node/pull/4615


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>