Followup r80375: let PreprocessorTest work on Preprocessor_Hash etc as well as Prepro...

Followup r80375: let PreprocessorTest work on Preprocessor_Hash etc as well as Preprocessor_Dom

Using same technique as ApiExpandTemplates to serialize the object tree back to XML, rather than asking for the DOM implementation's internal XML return function.
Have to also perform normalization on the test cases, as they aren't normalized to what libxml2 serializes. :P

Note that there are 4 test failures currently with Preprocessor_Hash, as it makes a separate <equals> element around = which doesn't appear to be in Preprocessor_Dom's output.