Deprecate Commons Testing

The module Commons Testing seems to be rather old and not too actively maintained. For most (if not all) of its classes there are nowadays better replacements:

For the individual packages I consider the following alternatives the better choices

1. o.a.s.commons.testing.integration: Teleporter or the org.apache.sling.testing.clients should be used preferably instantiated through Sling HTTP testing rules.
2. o.a.s.commons.testing.jcr: jcr-mock should be used instead
3. o.a.s.commons.testing.junit: the retry logic from the Sling HTTP Testing Client should be used.
4. # o.a.s.commons.testing.junit.categories: o.a.s.testing.junit.rules.category.SlowRunningTest from Sling HTTP testing rules. should be used instead.
5. o.a.s.commons.testing.osgi: osgi-mock should be used instead
6. o.a.s.commons.testing.sling: sling-mock should be used instead
7. org.apache.sling.commons.testing.util: if really useful can maybe moved to sling-mock as well, but probably not that useful. Currently TestStringUtil is not used at all and JavascriptEngine is only used from o.a.s.commons.testing.integration.

In addition there are certain limitations in that module which are not easy to fix:

1. o.a.s.commons.testing.jcr uses Jackrabbit 2 only and never Oak, that means that the ITs based on those classes are pretty far away from what we ship now in Sling.
2. o.a.s.commons.testing.jcr is currently not compatible with Java 9 (https://issues.apache.org/jira/browse/SLING-7159)

The according discussion around this can be found in https://www.mail-archive.com/dev@sling.apache.org/msg70983.html.