Showing disabled tests is implemented by a new member function on the
TestEventListener interface (which is responsible for printing
testing output). The new function is called OnTestSkipped and it is
invoked when a disabled test is encountered.
The PrettyUnitTestResultPrinter has the canonical implementation of this
new function. The BriefUnitTestResultPrinter and the
EmptyTestEventListener get a nullary implementation. The
JsonUnitTestResultPrinter and XmlUnitTestResultPrinter
inherit that trivial implementation from the EmptyTestEventListener.
