Add a way to exclude individual testharness tests

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

Start by reading the testharness.js context and the issue's examples involving individual test() calls in existing feature-aspect.html files. Define how an individual test can be excluded from an active focus area without moving it or making it conditional. Done means the approach addresses the stated conflict while preserving normal specification development.