[build] ensure we have core/target/test-classes and test/target/test-classes for some tests

maven/jruby/src/it/extended and maven/jruby-complete/src/it/extended depend
on test classes from core/ and test/ so running ```mvn -Pcomplete,test```
or ```mvn -Pmain,test``` will ensure those extra test-classes and will trigger
the test runs for those artifact

Sponsored by Lookout Inc.
added regular tests for jruby-complete and jruby-core/jruby-stdlib artifacts

some tests are not working and get omitted - https://github.com/jruby/jruby/issues/2215
and one test got a little workaround for https://github.com/jruby/jruby/issues/2216

Sponsored by Lookout Inc.