Consolidate the use of stubs

We have two separate StubOutForTesting objects when running tests. One
that is instantiated directly in the moxstubout fixture and one that
mox has for itself.

When a method is stubbed, the overridden method is kept in a cache in
the StubOutForTesting instance. When calling UnsetAll() on this
instance, the order of the cache is reversed and the original methods
are put back in place. This means if you have SomeObject with a method
called actual_method and you stub that out with

    stubs.Set(SomeObject, "actual_method", fake_method)

you end up with a this tuple in the cache:

    (SomeObject, "actual_method", SomeObject.actual_method)

If you stub it out again

    stubs.Set(SomeObject, "actual_method", fake_method2)

this tuple is appended to the cache:

    (SomeObject, "actual_method", fake_method)

When calling UnsetAll(), the cache is reversed and becomes:

    [(SomeObject, "actual_method", fake_method),
     (SomeObject, "actual_method", SomeObject.actual_method)]

So first, SomeObject.actual_method (which currently is fake_method2) is
set to fake_method, and then set to the original
SomeObject.actual_method medthod.

If, however, you stub out the same method using both mox and our own
StubOutForTesting object, one of the caches will have

    (SomeObject, "actual_method", SomeObject.actual_method)

and the other will have:

    (SomeObject, "actual_method", fake_method)

At this point, restoring everything back to the original state depends
on these StubOutForTesting objects getting cleaned up in the correct
order. If it's done in the wrong order, SomeObject.actual_method
(currently set to fake_method2) will get reset to the original
SomeObject.actual_method and subsequently get "reset" to fake_method
which makes life miserable for subsequent tests that might rely on the
real implementation.

This change stops us from having two separate StubOutForTesting objects
by simply using mox's all the time.

Fixes bug #1239898

Change-Id: I3a768eca3432e6df09c87bd86bffa9763aabfbed