Fix memory leak in NilTest

The NilTest is an AbstractCustomValidator, which is created via an OSGi service. When validating the loaded file, Xtend factory methods were used, which add instances to a cache. This is done for each XML node that is validated.

However as the NilTest is a service, it is never destroyed during the runtime of the process. As a result the validation cache is never cleared after a file is closed. This leads to a memory leak as we continously add new entries to the cache.