Revision,Storage: Widen @covers tags in tests

Given all called same-class methods are de-facto and liberally claimed,
and that we keep the coverage limited to the subject class, it maintains
the spirit and intent by listing the class as a whole instead.

PHPUnit offers a more precise tool when you need it (i.e. when testing
legacy monster/god classes), but for well-written code, the
class-wide tag is exactly what you want.

We lose useful coverage and waste valuable time on keeping tags
accurate through refactors, especially private functions (or worse,
forget to update it).
Tracking tiny per-method details wastes time in realizing (and
fixing) when people inevitably don't keep them in sync, and time
lost in finding uncovered code to write tests to realize it was
already covered but "not yet claimed".

Ref https://gerrit.wikimedia.org/r/q/owner:Krinkle+is:merged+message:Widen

Change-Id: If90fc5285a067ec8f706d87b2ba1ae85020e2ba0