Use Jest for testing

[Turbo87]

... instead of Mocha and Chai. And this introduces snapshot tests to assert on the coverage-summary.json results.

[rwjblue]

I think will need a rebase to account for the tests in #290.

[Turbo87]

rebase done ✅

[rwjblue]

I think the test-packages/index-test.js also needs to be updated to Jest syntax:

FAIL test-packages/index-test.js
  ● index.js › _getIncludes › _getIncludesForInRepoAddonDirectories › for an app with an inrepo addon under a custom path › instruments the inrepo addon

    TypeError: Cannot read property 'deep' of undefined

      510 |         it('instruments the inrepo addon', function () {
      511 |           const includes = Index._getIncludesForInRepoAddonDirectories();
    > 512 |           expect(includes).to.deep.equal([
          |           ^
      513 |             'my-app/utils/my-covered-util.js',
      514 |             'my-app/utils/my-uncovered-util.js',
      515 |             'my-in-repo-addon/utils/my-covered-util.js',

[Turbo87]

oh, whoops, totally missed that part 🤦‍♂️