Fix 3-1-stable to work with Mocha >= v0.13.0

A) Update code in ActiveSupport which monkey-patches Test::Unit to
include Mocha bug fix.

A bug was fixed [1] in Mocha's integration with Test::Unit, but this
monkey-patching code was copied before the fix. We need to copy the
fixed version.

The bug meant that an unexpected invocation against a mock within the
teardown method caused a test *error* and not a test *failure*.

B) Fix for Test::Unit/Mocha compatibility.

Mocha is now using a single AssertionCounter which needs a reference to
the testcase as opposed to the result.

This change is an unfortunate consequence of the copying of a chunk of
Mocha's internal code in order to monkey-patch Test::Unit.

C) Avoid a Mocha deprecation warning.

[1]
https://github.com/freerange/mocha/commit/f1ff6475ca2871f2977ab84cabbbfe2adadbbee6#diff-5
Merge pull request #8871 from freerange/3-1-stable-with-mocha-fixes

Fix 3-1-stable to work with Mocha >= v0.13.0