This only works with mocha v0.13.0 or later.
Note that this also fixes a few subtle bugs present in the current
* Mocha was raising a `MiniTest::Assertion` instead of a
`Mocha::ExpectationError` as intended. The latter is not recognized by
MiniTest as an assertion failure and so it is recorded as a test
*error*, not a test *failure* as it ought to. This leads to
potentially confusing output in the test results.
* Mocha verification should happen as part of the test. The verification
of expectations is equivalent to a set of assertions. These assertions
should happen as *part of* the test so that they have a chance to
cause the test to fail, and not just as part of the teardown. Also if
an assertion fails during the test, then there is no need to verify
expectations, because only the first assertion failure is normally
reported and all subsequent bets are off.
* Expectation verification should be counted as an assertion. Mocha
cannot record each expectation verification as an assertion, because
we weren't passing in an assertion counter to `#mocha_verify`.