Change snapshot header for Jest mocks to MockFunction

[mjesun]

Do you want to request a feature or report a bug?
That's gonna be a feature.

What is the current behavior?

Snapshots containing mocks are serialized as Object with calls + name. While this is correct, an object with an equivalent structure could make the snapshot pass (see #4829). For instance:

{
  calls: [],
  name: 'jest.fn()',
}

Would make the test pass, despite this is not a jest.fn().

What is the expected behavior?

We will change the header signature from Object to MockFunction, which will make impossible to accidentally match with an object.

[SimenB]

#4836

[JustJelena]

@mjesun which version did this change first go in, please?
I can't find #4835 or #4636 in the change log https://github.com/facebook/jest/blob/master/CHANGELOG.md

Many thanks in advance!
Jelena

[SimenB]

@JustJelena in Jest 22, look for #4668 in the changelog. The change discussed in this issue landed between alphas, so conceptually it's part of #4668

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