Update: add regression tests for Git.spawn env issues

[BYK]

Summary

Regression tests for #3742 and follow up to #3743.

Test plan

Tests shall pass.

[cpojer]

Why do you like braces and return statements so much? :P

[bestander]

src/utils/__mocks__/child.js gets compiled into lib on build, I wonder if it affects Yarn in any way.

Also jest complains

jest-haste-map: duplicate manual mock found:
Module name: child
Duplicate Mock path: /Users/bestander/work/temp/yarn/lib/util/mocks/child.js
This warning is caused by two manual mock files with the same file name.
Jest will use the mock file found in:
/Users/bestander/work/temp/yarn/lib/util/mocks/child.js
Please delete one of the following two files:
/Users/bestander/work/temp/yarn/src/util/mocks/child.js
/Users/bestander/work/temp/yarn/lib/util/mocks/child.js

jest-haste-map: duplicate manual mock found:
Module name: child
Duplicate Mock path: /Users/bestander/work/temp/yarn/lib-legacy/util/mocks/child.js
This warning is caused by two manual mock files with the same file name.
Jest will use the mock file found in:
/Users/bestander/work/temp/yarn/lib-legacy/util/mocks/child.js
Please delete one of the following two files:
/Users/bestander/work/temp/yarn/lib/util/mocks/child.js
/Users/bestander/work/temp/yarn/lib-legacy/util/mocks/child.js

[bestander]

I'll revert this one for today