Refactoring: Refresh testify's implementation of assert tests

It's time to refresh the testify implementation of our `assert` package. I originally chose to embed it into the `mspec` package for several reasons:
- To remove dependencies from the code.
- The author said that `testify` wasn't stable yet and could change at that time.
- I didn't want to increase complexity with package management, just to run tests.
- Some assert tests could not be ported as-is.
- Needed to override their error handling.

That last one was the key reasons. I could have mocked up a fake context to override their version of `testing.T`, but that felt like a lot of bloat given the other concerns above. So in the end, I just included a slightly modified version with `mspec`.

Since then, I've found a few small bugs and even a broken test or two with that old `testify` version. I've also noticed additional comparisons that testify now has.

So, it's time to refresh this package with their latest. And also, review the license to see if it has changed.