This way, it can work even with BlockHound on JDK8, which also uses
ByteBuddy and thus was in conflict.

Kind of solves
#1060, but since
now the debugging routine depends on BlockHound, where, it seems,
the same problem was not fixed, the original cause for concern
probably still stands.

Integration with BlockHound revealed that several tests were
performing blocking operations such as Thread.sleep in
coroutine context `Dispatchers.DEFAULT`.

Also, some tests hanged because the exception that notified about
blocking calls were wildcard-matched.

Before, the tests only knew that the `park` native method was moved
to `jdk.internal.misc.Unsafe` from `sun.misc.Unsafe`. However, in
JDK 11, there is no `sun.misc.Unsafe` at all, it seems that it
has been moved completely.

This change is beneficial for this feature set because BlockHound
puts its `$$BlockHound$$_park` method to `sun.misc.Unsafe` or to
`jdk.internal.misc.Unsafe` depending on the JDK version. Also,
judging by BlockHound's code
https://github.com/reactor/BlockHound/blob/091d7b139479b1c41eea59baa23389d673fdf73b/agent/src/main/java/reactor/blockhound/BlockHound.java#L177-L187,
`forkAndExec` had been moved, too, but it is not used in tests.

Now it relies on the ServiceLoader mechanism to load the
integration: if the user installs BlockHound and
`kotlinx-coroutines-debug` is in the classpath, then BlockHound
will know to detect blocking calls in scenarios that forbid it.

The feature that we need was released, so no need to incclude the
snapshot repositories in the build.

* publication-validator is renamed to integration-testing;
* Each test is now in a separate source set, which allows for more
  flexibility in their configuration; for example, failing to set
  `dryRun=true` doesn't prevent tests other than NPM to run, and
  it is possible to run the tests (and their dependencies)
  separately.

Co-Authored-By: Sergei Egorov <bsideup@gmail.com>