feat: add support for generator functions in specs

[SimenB]

Summary
Fixes #2666

Easy enough to do, and brings some more feature parity with ava (https://github.com/avajs/ava/blob/965cbc65d82b12019cdf6a3257b3198dd4181b79/readme.md#generator-function-support).

Also, I was bored, and this was the lowest hanging fruit I could find with some people behind it :P I didn't update the docs much as async-await is to be preferred. Can point it out if we want it, though.

(Feel free to close this if you really don't want it, but I don't see the harm, tbh (6 lines of code))

Test plan
Integration test added. I didn't see any unit tests for the promisification happening to it already.

[codecov-io]

Codecov Report

Merging #5166 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5166      +/-   ##
==========================================
+ Coverage   60.64%   60.65%   +0.01%     
==========================================
  Files         201      201              
  Lines        6695     6695              
  Branches        4        3       -1     
==========================================
+ Hits         4060     4061       +1     
+ Misses       2634     2633       -1     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-jasmine2/src/jasmine_async.js	33.33% <ø> (+16.66%)	⬆️

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[cpojer]

Good work and Merry Christmas!

[jbdemonte]

@SimenB Any possibility to support generators in mockReturnValue (Resolve/Once)?
Idea would be to get a different value after each call to a jest.fn based on a generator, and without having to provide an implementation like:

      function* generator() {
        yield 1;
        yield 2;
        yield 3;
      }

      const iterator = generator();

      const fn = jest.fn().mockImplementation(() => iterator.next().value);
      expect(fn()).toEqual(1);
      expect(fn()).toEqual(2);
      expect(fn()).toEqual(3);
      expect(fn()).toEqual(undefined);

A cool handler would be a kinda:

jest.fn().mockReturnIteratedValue( generator() )

[github-actions]

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