Run the tests in registerJavaFunction and registerJavaUDAF only when test classes are complied

If you do a plain package with sbt:

./build/sbt -DskipTests -Phive-thriftserver clean package
./run-tests --python-executable=python3 --testname="pyspark.sql.udf UserDefinedFunction"

The doctests in registerJavaFunction and registerJavaUDAF fail because it requires some classes from the test compilation.

We should skip them conditionally