Syck specific YAML parser tests can only run with the old engine.

Ruby 1.9 moved to using the Psych engine as the default YAML engine, replacing
the older Syck based YAML engine. They did this without any regard to parse
tree compatibility, or abstraction over the underlying tool.

Consequently, the test that gets very intimate with the encoding format for
the parsed YAML tree is unable to function with the modern engine - it tries
to interact with a pile of methods that just don't match.

This commit disables that test unless the Syck parser is in use. Really, the
entire test should be an external, abstracted acceptance level test.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>