Convert (most of) the test data files into genuine XML. A handful still

are not, due mainly to the lack of support for XML character entities
(e.g. & => & ).  This will make it easier to validate test files using
tools like xmllint, as well as edit and view them using XML tools.

tests/data/test20

@@ -1,3 +1,4 @@
+<testcase>
 <info>
 <keywords>
 non-existing host name
@@ -29,3 +30,4 @@ non-existing-and-weeeeeeirdname.
 6
 </errorcode>
 </verify>
+</testcase>