JBPM-4069 XmlBPMNProcessDumper.dump() misses conditionExpression in sequ...

Test case and fix.

SplitNodeBuilder swaps the Constraint without complete field mapping during ProcessBuilderImpl.buildProcess() and it results in this problem. XmlProcessReader.read() doesn't cause this. So I added new XMLBPMNProcessDumperTest class instead of adding a test method to BPMN2XMLTest.