Add a way to exclude individual testharness tests

[andruud]

The following two things are in conflict with each other:

• An active focus area wanting to govern a test (file), freezing its scope.
• A WG operating as normal, developing/extending a specification, necessarily increasing scope.

When some feature aspect is extended in a specification, there is often an existing feature-aspect.html testharness WPT where it makes the most sense to place a test (test()) for this extension. However, doing so would inadvertently increase the scope of the active focus area.

We currently have these choices:

• Move the test for the new extension away from its natural habitat, i.e. a new file, artificially. (Undesirable)
• Exclude the whole test from the active focus area, possibly allowing a minor extension to "poison" all of feature-aspect.html. (Undesirable)
• Run the new test (test()) conditionally, based on feature detection (CSS.supports(), etc). (Undesirable, though arguably the least bad of these three.)

This might be a general issue that goes beyond testharness.js, but I think most of the pain would go away if we could somehow mark individual test()s as excluded. cc @foolip