Add a way to test different markup parser flags

For now, we are mainly interested in G_MARKUP_TREAT_CDATA_AS_TEXT.
This commit makes markup-parse look for expected output files with
the extension .cdata-as-text in addition to .expected, and compares
the output of parsing with G_MARKUP_TREAT_CDATA_AS_TEXT against
them. markup-parse --cdata-as-text foo.gmarkup can be used to produce
such expected output.
Add a few tests for G_MARKUP_TREAT_CDATA_AS_TEXT

Related to
https://bugzilla.gnome.org/show_bug.cgi?id=725433