Implement addMatchers shortcut on jest

[yungsters]

This will make it easier to use arrow functions when writing unit tests that have custom matchers:

beforeEach(() => {
  jest.addMatchers({
    toBeOne() {
      return this.actual === 1;
    }
  });
});

[zpao]

Don't want to write the docs?

[zpao]

Seems fine. Might be worth adding a comment in the code that it will need to be updated if we upgrade to jasmine2. http://jasmine.github.io/2.0/upgrading.html

[DmitrySoshnikov]

Shall we go with this PR? Also, we have the process of upgrading to jasmine2 in #330.

[DmitrySoshnikov]

OK, seems it was merged internally already, so we can add it here.

[github-actions]

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