Introduce conditionally disabling tests

[cspray]

Right now you can only disable a test statically, sometimes you might want to disable a test only if certain conditions are met. For example, if the code under test is optional for your application and requires a certain extension the test suite should disable any tests that require that extension if it is not available. This is currently not possible. We should avoid introducing a TestCase::markDisabled... it requires all of the hooks and test processing to still be invoked when we could, and should, be skipping past these potentially costly, error-triggering procedures. Instead we should introduce a #[DisabledIf($method, $reason)] Attribute. The $method should be a public method on the class it was annotated on. If it returns true the test will be disabled otherwise it will run as normal.

[cspray]

I decided to move this out of the 0.6 release because getting this implemented is going to require changing around a fair amount of how we think of disabled tests. Statistics for pre running will need to be adjusted because we no longer can reliably tell whether or not a given test/case/suite is disable until runtime. I believe that this feature is important enough and has enough impact on the existing codebase that the implementation needs more thought.