tests: switch to XML::TreePP for testing Atom feeds
XML::Feed pulls in a lot of dependencies, some of which XS.
That makes testing with blead or any non-OS-supplied Perl
installations more time consuming and more difficult because
of the need to have development headers and libraries for
libexpat1 or libxml2.
Performance from libexpat1 or libxml2 for our small tests cases
isn't relevant, either, and the pure Perl XML::TreePP seems up
to the task. It's also available in CentOS 7.x, FreeBSD 11.x,
and Debian, at least.