test: write to tmp dir rather than fixture dir

test-fs-realpath.js was writing files to the fixture dir. This changes
it to use the temp directory instead. This also replaces some of the
string concatenation for paths with uses of path.join() and
path.relative().

PR-URL: https://github.com/nodejs/node/pull/4489


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>