Introduce API to inject main dispatcher

[qwwdfsad]

Otherwise, Android users should inject main dispatcher into their classes to use Unconfined in tests. This is pretty ugly, especially when Dispatchers.Main is already here.

Proposed solution:
Introduce module kotlinx-coroutines-core-test which overrides Main dispatcher and API to inject any dispatcher as Main.

So users can use launch(Dispatchers.Main) and in tests all they need is to add a dependency to kotlinx-coroutines-core-test and use MainDispatcherInjector.inject(dispatcher: CoroutineDispatcher).

[pull-vert]

I think providing a separate module 'kotlinx-coroutines-core-test'' is really important, to provide Coroutines users a dedicated module with some useful testing tools such as TestBase or TestCoroutineContext (this last one provides useful delay testing features, but will need some changes, also it shouldn't be in 'kotlinx-coroutines-core').

[qwwdfsad]

Yes, that's the plan. The plan is to provide more testing utilities (sequential execution validation, virtual time, debug capabilities etc.)

[BoxResin]

This feature is really needed!