kotlinx-coroutines-test modules

[pull-vert]

It would be nice to have separate modules dedicated to Tests, so external libraries can use it to test Coroutines by importing it like this in gradle (example for jvm) :
testCompile "org.jetbrains.kotlinx:kotlinx-coroutines-test:$kotlinx_coroutines_version"

This module would provide TestBase class, containing runTest builder and useful methods to track good execution in coroutines.

A MPP would be perfect (common, js and jvm, and why not native too).

Edit : it would be nice if this module can include TestCoroutineContext too (#297)