readonly suite, nodeValue setting tests, entity tests

Added a suite containing all tests that do not attempt document modification (readonly.xml)  for convienient testing of read-only DOM implementations (apparently Xalan-J).  This undercovered several tests that had indicated that a loaded document would not be modified when it actually would have been.

I've also added tests or assertions (to documentcreate* tests) that cover setNodeValue for all derived types and covered Entity.getSystemId and Entity.getNotationName.

Mozilla seems to be very close to supporting null strings (see http://bugzilla.mozilla.org/show_bug.cgi?id=69468) will evaluate a daily build in the next few days.