oxml: add tests for oxml_parser
oxml: add test for register_custom_element_class()
oxml: extract serialize_part_xml()

A general purpose function, oxml_tostring(), was being used to
serialize the XML for a part before saving. This placed a lot of
unnecessary lxml API choices in the part code (in the form of parameter
choices on oxml_tostring(). Creating a special-purpose function for
this single job greatly simplifies the interface and conforms to the
"do one thing" principle.
oxml: move oxml_tostring() to unit serialize_xml()

oxml_tostring() is now used only by unit tests. Move to unitutil as
serialize_xml() and remove unused parameters.
oxml: mv oxml_fromstring() parse_xml_bytes()
test: add mocking util functions to unitutil
oxml: add unit tests for parse_xml_bytes()
oxml: extract parse_xml_file() to unitutil
test: add unit tests for pptx.oxml.ns
oxml: add tests for oxml.core.get_or_add()

* Refactored get_or_add() to only operate at direct child level, since
  that was all it was being asked to do so far.

* Moved fixtures into test classes as an experiment, might be cleaner
  for cases like this one. We'll see how it goes as more tests are
  added.
oxml: add tests for oxml.core.child()

Moved shared fixtures to module level.
oxml: add tests for oxml.core.Element()

Also did some identifier renaming in Element to clarify.
oxml: add tests for oxml.core.SubElement()