Adding async versions of mockReturnValue & mockreturnValueOnce

[m4n3z40]

Summary

I find myself doing the following very often when testing async code:

// ...
const asyncFn = jest.fn().mockReturnValue(Promise.resolve(result)):

// or...

const asyncFn = jest.fn().mockReturnValue(Promise.reject(err)):
// ...

This PR adds four utility methods to MockInstance, automating the creation of async mock functions:

// ...
const asyncFn = jest.fn().mockResolvedValue(result):

// and...

const asyncFn = jest.fn().mockResolvedValueOnce(result):

// and...

const asyncFn = jest.fn().mockRejectedValue(result):

// and...

const asyncFn = jest.fn().mockRejectedValueOnce(result):

// ...

Test plan

NA (Not a bug)

---

This PR means to gather info on usage first, if people find this useful I'll continue to add the docs to this feature and updating the changelog as needed.

Thanks!

[codecov-io]

Codecov Report

Merging #5318 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5318      +/-   ##
==========================================
+ Coverage   61.29%   61.33%   +0.03%     
==========================================
  Files         205      205              
  Lines        6917     6924       +7     
  Branches        3        3              
==========================================
+ Hits         4240     4247       +7     
  Misses       2676     2676              
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-mock/src/index.js	86.49% <100%> (+0.31%)	⬆️

---

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[SimenB]

@m4n3z40 Thanks for the PR, and sorry about the slow review! I started writing it, but I just ended up fixing my code nit in order to ensure it worked like I wanted. I rebased your branch and added my change, the assertions read a bit nicer this way 🙂

[SimenB]

Please update the changelog and mock documentation.

[m4n3z40]

Awesome. Wasn't aware that all jest features were available in it's own test suite. Very neat, thanks.

Added changes to CHANGELOG and docs.

[m4n3z40]

Just noticed eslint is failing because of the async examples I've added. :(

[akira28]

The doc about mockFn.mockRejectedValueOnce(value) says that is sugar for jest.fn().mockReturnValueOnce(Promise.resolve(value));
Shouldn't be the sugar for jest.fn().mockReturnValueOnce(Promise.reject(value)); ?

[SimenB]

Fixed in #5373, isn't it?

[akira28]

uups, didn't see it :)

[SimenB]

No worries, thanks for checking!

[github-actions]

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