Test case for failing regular expressions in YAML.pm

I added some test cases demonstrating the bug listed at https://bugzilla.redhat.com/show_bug.cgi?id=192400 and https://rt.cpan.org/Public/Bug/Display.html?id=21077.

Basically YAML dies on large single or double quoted strings (~32k characters) because of two relevant regular expressions causing deep recursions, at least on my Perl 5.12. These test cases demonstrate the behaviour.

Unfortunately my original attempt to fix the issue was not successful. I have to leave it to more knowledgeable people.