Create temporary pathwatcher test file in app directory.

Previously we were testing pathwatcher.watch by creating a file in
.meteor/local, but developers using Vagrant VMs often mount .meteor on a
different (non-network) file system, for Mongo's sake.

Writing the file directly in the app directory increases the likelihood
that it will interact with pathwatcher in the same way as other source
files the developer will be editing.
Run file watching tests with both pathwatcher.watch and fs.watchFile.

Running the two versions of the test in parallel is safe because they run
in separate processes, and a good idea because it cuts the time taken by
the test in half, from 36sec to 19sec.