Fix @covers for FileBackend

Fix @covers for FileBackend

It's up to the developer to choose the level of granularity they are
aspiring for in their test coverage. But that granularity level should be
reflected in @covers. When you have hundreds of lines of code testing a
single-line function (FileBackend::doOperation), that's a good hint that
something went wrong.

FileBackendTest is 2700 lines of code and appears to be aiming to test
the whole filebackend module. So I adjusted @covers to reflect that.

Change-Id: Iacd8cf475d1761c3b32c739983343619a9509d6b