test: split independent tests into separate files

Move ENOENT related tests out of general fs.watch() test file and into
its own file. This may help diagnose
https://github.com/nodejs/node/issues/3541.

PR-URL: https://github.com/nodejs/node/pull/3548


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