Export cleanup method for did-intersect mock

[elwayman02]

The test support util we created for did-intersect doesn't have a way to restore the mock after a test: https://github.com/elwayman02/ember-scroll-modifiers/blob/master/addon-test-support/did-intersect-mock.js

We should export a method that makes it easy to clean up after the tests and avoid leaking state.

We do something similar here: https://github.com/elwayman02/ember-scroll-modifiers/blob/master/addon-test-support/scroll-into-view-mock.js#L24

cc: @ygongdev