feat: add class for mocking Math.random

[ojeytonwilliams]

• feat: add RandomMocker class to mock Math.random
• docs: explain how RandomMocker can be used

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

This class lets us control Math.random in testing by replacing it with a pseudorandom number generator with a fixed seed.

[ilenia-magoni]

If this is made to be used in --before-user-code-- like in freeCodeCamp/freeCodeCamp#56151, there is a slight issue... helpers are not accessible in --before-user-code-- and --after-user-code--, there __helpers is not defined.

[ojeytonwilliams]

That's a good point - when using this you have to instantiate the RandomMocker in the test code. For example, the linked issue would no be able to use --before-user-code-- and, instead, would do this in the test:

let mocker = new __helpers.RandomMocker();
mocker.mock();
try {
  rollDiceBtn.click();
  const old = [...diceValuesArr];
  rollDiceBtn.click();
  assert.lengthOf(diceValuesArr, 5);
  assert.notDeepEqual(old, diceValuesArr);
} finally {
  mocker.restore();
}

[naomi-lgbt]

Don't mock Math.random(), it's trying its best and mocking it is mean 🙁