Run html5lib-tests fragment construction tests with an instantiated context node

[flavorjones]

As noted in #2691, the html5lib-tests fragment construction tests are being run using a string (tagname and optional namespace prefix) rather than an instantiated node. This is a hole in the test coverage that resulted in an undiscovered big fixed in #2692.

The desired outcome is to run the fragment tests twice: once with the tag name string, and once with a fully-instantiated node as the context.