In the current `Test` module there isn't an easy way to say "If this
dependency doesn't exist use this mock function instead." With this new
`useMocker` fluent function it becomes possible to tell the `TestingInjector`
to replace the undefined dependency with the automocking function. In doing so
classes that have a large number of dependencies no longer must be mocked one-by-one
. This is especially great when it comes to packages like `@golevelup/ts-jest`
which can create mocked dependencies of interfaces and classes with a single function.
I'm sure I've probably missed an edge case in here, so please let me know if you have
any major concerns.
