GMarkup: share some common code for closing tags

The code for dealing with </foo> and the second half of <foo/> was
largely duplicated.  We can share a lot of it by using a common
function.

This slightly changes the behaviour of the parser under error
circumstances: previously the parser would deal with '<foo/}' by first
issuing the end_element callback and then flagging the error due to the
unexpected character.  Now we will flag the unexpected character error
first, skipping the callback.

This behaviour change required modifying the testsuite.

https://bugzilla.gnome.org/show_bug.cgi?id=665634