Added the ability to configure the test executor to report test messages synchronously rather than asynchronously.  This is an important feature for toolsets that need to perform critical actions between individual test calls.
Moved the 'Arguments' property on XunitTestCase up onto ITestCase to make it available for toolsets.
Disabled custom trait discovery when working with toolsets that use static analysis to discover tests, as custom trait discovers can only function in runtime environments that can execute user code.