tests/htmltree: use better test file names

The new naming scheme makes it easier to correlate the test files
to the HTML spec. They relate to section 12.2.5.4.N of the WHATWG
spec. Specifically, they take the form:

    N-insertion_mode-rule-name.html

where:
    N is the number of the section corresponding to the insertion
      mode
    insertion_mode is the name of the insertion mode
    rule-name is a description of the rule being tested