Allow skipping tests based on certain condition

It would be useful if we could skip tests per OS, e.g. skip Level DB JNI tests for Solaris.

In order to achieve this, we will need a separate implementation for JUnit and TestNG (mainly because TestNG allows incorrectly spelled groups, like JUnit Categories, and JUnit fails the build on missing groups; so we can't have both).

TestNG implementation would require an AnnotationTransformer whereas JUnit a Rule. Plus proper annotations like @skipOnWindows or anything similar.