Update tests to be HTML version neutral

[castaway]

HTML::Tagset is working on HTML v5 support (by default), this patch updates Tree's tests so that they do not contain html4-specific content. See libwww-perl/HTML-Tagset#2

Unfortunately (I tried!) I failed to get dzil test to work, happy to test if someone can provide instructions.

Ended at:

castaway@insel /usr/src/perl/HTML-Tree $ (eval $(perl -Mlocal::lib=/usr/src/perl/libs/develop/perl5/); dzil test)
[DZ] building distribution under .build/94uQfMds1h for installation
[DZ] beginning to build HTML-Tree
[Git::NextVersion] Bumping version from 5.910 to 5.911
[Repository] Skipping invalid git remote github
[DZ] guessing dist's main_module is lib/HTML/Tree.pm
[PodWeaver] [AUTHOR] no repo at weaver/authors.pod line 36.
[PodWeaver] [AUTHOR] no repo at weaver/authors.pod line 36.
at inline delegation in Pod::Weaver::Section::Template for logger->log_fatal (attribute declared in /usr/src/perl/libs/develop/perl5/lib/perl5/Pod/Weaver/Role/Plugin.pm at line 58) line 18.