separatorPosition="prefix" fails to lay down a separator in any position within the grammar!

Created a test in daffodil-test/src/test/scala-debug/daffodil/section05/dfdl_xsdl_subset/TestDFDLSubset.scala. It's called "test_prefix". The original groupRef test was copied and modified to attempt to just parse each "Item" as a string delimited by "||".