Adds t.mock() API

[isaacs]

This brings into tap the standard mocking system we have been using
across the ecosystem of packages from the npm cli which consists into
hijacking the require calls from a given module and defining whatever
mock we want via something as simple as a key/value object.

It's a very conscious decision to make it a very opinionated API, as
stated in https://github.com/tapjs/node-tap#tutti-i-gusti-sono-gusti -
focusing only on the pattern that have been the standard way we handle
mocks.

It builds on the initial draft work from @nlf (ref:
https://gist.github.com/nlf/52ca6adab49e5b3939ba37c7f0fc51c6) and
initial brainstorming of such an API with @mikemimik - thanks ❤️

Example:

t.test('testing something, t => {
  const myModule = t.mock('../my-module.js', {
    fs: { readFileSync: () => 'foo' }
  })

  t.equal(myModule.bar(), 'foo', 'should receive expected content')
})

Credit: @ruyadorno, @nlf
Reviewed-by: @isaacs
PR-URL: tapjs/tapjs#698
Closes: tapjs/tapjs#698