test: cleanup parallel/test-fs-realpath.js

Replaced .indexOf() with .includes() for more clarity as to
what it is doing.

Many of the instances of var changed to const.

Instances of assert.equal() refactored to assert.strictEqual() or
assert.ifError()

Removed the unlinkSync() call in the exit event handler because it
probably only handles files in the testing tmp directory so there is
no need to unlink them.

PR-URL: https://github.com/nodejs/node/pull/8769


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>