Support jUnit5 extension

[tasomaniac]

Support jUnit5 extension. It is super easy to do, the question is how to distribute it. A new module with new artifact? Or include them together? I also don't know what is the best practice.

Here is how it would look like

class MockServerExtension : BeforeEachCallback, AfterEachCallback {

    lateinit var server: MockWebServer

    override fun beforeEach(context: ExtensionContext) {
        server = MockWebServer()
        DispatcherRetainer.registerRetainer()
        DispatcherRetainer.resetDispatchers()
        server.start()
    }

    override fun afterEach(context: ExtensionContext) {
        server.shutdown()
        DispatcherRetainer.resetDispatchers()
        server.setDispatcher(DispatcherRetainer.queueDispatcher)
    }
}

[tasomaniac]

If we agree here, I would like to contribute this.

[JorgeCastilloPrz]

I think a new artifact would fit better IMO. hiroaki-junit5.

One example of other libs using the same approach, Spring: https://github.com/sbrannen/spring-test-junit5/blob/master/README.md#jitpack-with-gradle