upload: Simplify UploadStashTest by using getNewTempFile()

upload: Simplify UploadStashTest by using getNewTempFile()

The parent class has a built-in list that tracks these files and
deletes them in a teardown.

I was unable to reproduce a case where $path + '.' is created by
MediaWiki. As far as I can tell, no current code exists in core
that would do this. If it does exist, we can keep the tearDown()
override with just that second case, but I'm removing it for now,
given it doesn't appear to be used.

Change-Id: I5847b7b266f1393d983aeb4b115bc0ae000e3547