When writing unit tests for XML data, it is more convenient to compare
list of lines, instead of a single string without newlines.
This helps identifying which lines in the diff printed on the console
don't match the expected result.
So, getXML now returns a list of lines,
To allow passing the same list of lines to the complementary parseXML
function in the roundtrip tests, parseXML now also accepts a list of
strings, as well as a single string.
We also use unicode_literals, and ensure that if the test modules passes
unicode str, we first encode to UTF-8 before passing on to expat XML
parser. This is because on Python 2, expat only accepts bytes strings.
This should make the unit tests more readable, and it also prevents
failures where a changed fromXML() implementation fails to ignore
interspersed whitespace. This has recently caused some developer
nuisance; see https://github.com/behdad/fonttools/pull/367.
Instead of having a custom parser implementation, it would be nicer
to call the actual XMLReader. However, XMLReader has a deeply built-in
assumption that it is processing an entire TTX file. Changing this
assumption would certainly be possible, but it would need a re-write
of the XMLReader code; having a custom parser just for unit tests
seems less involved.
