feat: Reset mock extensions

[getsaf]

Currently, using Mock.extend will mutate the thing being extended. eg:

foo.ts

export const FOO = {
  bar: 'bar';
};

example.test.ts

import { FOO } from './foo';

describe('mutations', () => {
  it('can mock FOO.bar', () => {
    Mock.extend(FOO).with({bar: 'fake bar'});
    expect(FOO.bar).toBe('fake bar');
  });
  it('bleeds mock data across tests!!', () => {
    // This will FAIL if tests are run in definition-order because
    // FOO.bar still equals the mock!
    expect(FOO.bar).toBe('bar');
  });

This change will auto-reset mock extensions to their original values in an automatic afterEach.

[getsaf]

Thoughts

Should we make this configurable? As it is, this change could cause breakages in projects that "depend" on data-bleed across tests.