Make sure function mocks match original arity

[palmerj3]

Summary

As reported in #3997, Jest mocks currently do not have the same arity as their underlying functions. This PR attempts to fix that.

Test plan

Run unit tests locally.
Test in a local project using tests provided in issue #3997
Write unit test in jest-mock to ensure arity is set properly

[cpojer]

Thanks for sending a PR, including tests. I think to get this merged it needs to be correct and support a variable length. After 10, we could code-gen and use "new Function", what do you think?

[codecov-io]

Codecov Report

Merging #4170 into master will decrease coverage by 0.13%.
The diff coverage is 34.28%.

@@            Coverage Diff             @@
##           master    #4170      +/-   ##
==========================================
- Coverage   60.55%   60.42%   -0.14%     
==========================================
  Files         196      196              
  Lines        6776     6809      +33     
  Branches        6        6              
==========================================
+ Hits         4103     4114      +11     
- Misses       2670     2692      +22     
  Partials        3        3

Impacted Files	Coverage Δ
packages/jest-mock/src/index.js	84.67% <34.28%> (-7.44%)	⬇️

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[palmerj3]

@cpojer I did attempt this several times. I'm not sure it's possible.. also this is very similar to how sinon solves this issue: https://github.com/sinonjs/sinon/blob/master/lib/sinon/spy.js#L61

[cpojer]

We could try-catch for this and fall-back to the solution that doesn't have proper arity. It means it'll work properly in most environments, and fall back to a working solution at least in an environment without eval.

[palmerj3]

@cpojer perhaps the default case should be the same as the 0 case. This way if it's out of range, as you say, it would return a 0 length function like it does right now.

[palmerj3]

@cpojer let me know if my current solution works for you or if I need to make other changes. I'm not sure a dynamic case will work... but if you could point me in the right direction I'm willing to try.

[cpojer]

Works for me, thank you!

[github-actions]

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