tests: switch from tmpnam to mkstemp

Use mkstemp to generate a temporary file in a safer way than tmpnam,
and placed in the temporary home created for the tests already.
xmltool: move testmod_xmltool.c to the top-level

No functional changes.
xmltool: change directory of tests data

Move the XML test data from the tests subdirectory to a new xmltool
one, installing it as a whole together with the other test data;
adapt testmod_xmltool and test_validate.sh to the new paths of the
test data.

This mimics how other plugins handle test data.
xmltool: remove now empty tests subdirectory
Merge pull request #45 from pinotree/xmltool-tests

xmltool plugin: fix handling of the test data
Merge pull request #44 from pinotree/tmpnam

tests: switch from tmpnam to mkstemp