Moved isIgnored-check from FrameworkMethod to BlockJUnit4ClassRunner -
the method overrides an empty default implementation in ParentRunner.
Draft: Changed implementation for detecting when a class has no tests to
run: Replaced special case object by an abstract Statement that is aware
of the children it will run. This allows us to evaluate whether a
statement has any tests to run using the isIgnored(T) method added in
the previous commit - any further handling of ignored tests will be done
by the actual implementation. ParentRunner just takes care of not
executing the BeforeClass/AfterClass methods and ClassRules if no tests
are remaining.