Integration tests

This is a supplementary project that provides integration tests.

The tests are the following:

MavenPublicationValidator depends on the published artifacts and tests artifacts binary content and absence of atomicfu in the classpath.
CoreAgentTest checks that kotlinx-coroutines-core can be run as a Java agent.
DebugAgentTest checks that the coroutine debugger can be run as a Java agent.
smokeTest builds the test project that depends on coroutines.

The integration-testing project is expected to be in a subdirectory of the main kotlinx.coroutines project.

To run all the available tests: cd integration-testing + ./gradlew check.