Fix XUnit output with Gecko

the toString() function is not called by gecko because a DOMNode
does not inherits from Object. We should call a specific function
to serialize the content of the XML.
updated xunit tests to refect use of getSerializedXML
added clitest for XUnit report generation
Merge pull request #759 from n1k0/fix-gecko-unit

Fix XUnit output with Gecko (@laurentj) - closes #753 #754