refactor: Several small changes

[fb55]

This is a bundle of several small changes that came up with html5lib/html5lib-tests#147

Overview over the changes:

• reconsumeInState will now call the state directly, instead of retreating.
• We knew in several places that we wanted to get to the template end logic of the in head insertion mode. By factoring out that insertion mode, we can skip some steps.
• Added a test for serialising unknown nodes (which should lead to an empty string)
• Test indices were previously global, which resulted in ignores failing after an unrelated test was added. Now, test indices are local.
• Simplify a conditional statement to a ternary (following the Principle of Most Restrictive Production)