Some contribs depend on core tests to be compiled and fail when ant clean was done before

If you do "ant clean" on the root level of Lucene and then go to e.g. contrib/queryparser (3.x only) or contrib/misc (3.x and trunk) and call "ant test", the build of tests fails:

• contrib/queryparser's ExtendedableQPTests extend a core TestQueryParser (3.x only, in module this works, of course)
• contrib/misc/TestIndexSplitter uses a core class to build its index

To find the root cause: We should first remove the core tests from contrib classpath, so the issue gets visible even without "ant clean" before. Then we can fix this.