Add a serializer for jest.fn to jest-snapshot

[SimenB]

Summary
This adds a serializer to be able to easily assert on the calls of a mock.

This is very easy to implement in userland, so I understand if it's not merged 🙂 But it's been useful for me, so maybe it'll be useful for others?

(I started creating a separate module for this, but I got annoyed that I couldn't use mock.getMockName(), so here's a PR)

Test plan
New tests added

[cpojer]

This is clever, I like it. Let's do it.

[codecov-io]

Codecov Report

Merging #4668 into master will decrease coverage by <.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #4668      +/-   ##
==========================================
- Coverage   57.13%   57.13%   -0.01%     
==========================================
  Files         194      195       +1     
  Lines        6565     6569       +4     
  Branches        3        3              
==========================================
+ Hits         3751     3753       +2     
- Misses       2814     2816       +2

Impacted Files	Coverage Δ
packages/jest-snapshot/src/plugins.js	100% <100%> (ø)	⬆️
packages/jest-snapshot/src/mock_serializer.js	50% <50%> (ø)

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[github-actions]

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