test: add support for NODE_TEST_DIR on a separate mount point

Linux permits a filesystem to be mounted at multiple points, but
`fs.renameSync` does not work across different mount points, even if the
same filesystem is mounted on both.
This fixes failing tests when NODE_TEST_DIR mount point is different
from the one on which the tests are executed (E.G. on a separate
partition).

Ref: http://man7.org/linux/man-pages/man2/rename.2.html

PR-URL: https://github.com/nodejs/node/pull/21552
Refs: http://man7.org/linux/man-pages/man2/rename.2.html


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>