Add the file name to assertion messages.

This is to have a better signal of which case is failing for the
_multipleFiles test cases.

Bug: 158502950
Test: atest CtsOsTestCases:android.os.cts.FileObserverTest#testFileObserver_multipleFilesFull
Change-Id: If0d0c701a4760079424b512203d55022ace1d6ef