Xunit1 start message for skipped test

The xunit1 API doesn't notify test start for skipped tests (unless you use a custom Fact). This means you don't get the right notifications through to the xunit2 API. This change ensures you get proper notifications for skipped tests - the previous test case is finished, a new test case is started, and a new test is started.

Note that this change is built on top of #72, since the tests are shared, and it means that the test method is also correctly started and finished, if appropriate.