Untie from Jest

It would be nice to be able to use Test Mule with other test runners, like Ava or Mocha. Test Mule is slightly tied to Jest, and very tied to Jest's `expect`. We should test to see if Test Mule can be used in other test runners by loading Jest's `expect` into the other runners, and/or look into untying Test Mule from `expect`.

Start by reviewing Test Mule's integration with Jest and Jest's `expect`. Try using Test Mule with Ava or Mocha, including loading Jest's `expect`, and compare that with investigating whether the dependency on `expect` can be removed; done means establishing a viable path for use with other test runners.

Written by the indexing model from the issue text.