Support a nicer way of mocking shim modules when using dojo/testing

[matt-gadd]

Enhancement
Currently their is no canonical way to mock @dojo/shim modules. This is a little annoying when wanting to mock things such as fetch and requires messing with the global.

Perhaps inside our shim modules we can have a wrapper that wraps the shim itself and when a test has flag is enabled provides an additional export that allows you to mutate it.

For example something like:

import fetch, { replace } from '@dojo/framework/shim/fetch'
replace(myMockFetch);

where replace (not attached to the name), is only available with the test has flag. This would allow you to swap the implementation.

[maier49]

With the PRs here, in cli-test-intern, and in cli-build-app merged this should be done.