Monkey patch `Date.prototype` getTime and valueOf methods and `Date.now`
method in _.debounce and _.throttle test cases. Provide additional test
case asserting the outputs before and after monkey patching. Remove the
reference to _.now method in test cases as the underlying implementation
of library methods don't reference the methods from the `_` object but
rather call dependent methods directly.

✅ Closes: #2883

Update the assertion comments in `_.throttle` and `_.debounce` unit tests,
in order to prevent confusion and add more brevity to the test case messages.

I just found out that RawGit will be shutting down. The maintainer
already announced this in 2018; we have been very lucky that the docs
continued working until now.

Because the RawGit links should change to Statically in here, too.

Rollup v1 accepts our rollup configuration, it just produces
incorrectly factored bundles for Node.js. The factoring does not
matter for the tests.

This proved necessary in order to preserve all aliases in the new
Node.js 12+ ESM entry point.