Test guard for HTML level 1 only tests

There were certain breaking changes in DOM L2 HTML where the correct behavior for L2 is different than the behavior for L1.  I just added an additional attribute to the <hasFeature> test prerequisite so that you can define a test that is valid only if a particular feature is not supported.

For the upcoming DOM L1 HTML tests where the L2 behavior is different and the L1 test should fail for L2 supporting implementations (and only tests like that), do something like:

<test>
    <metadata/>
    <!--  this test does not apply when the implementation supports HTML L2   -->
    <hasFeature feature="HTML" version="2.0" value="false"/>
    <var.../>
</test>