Stop testing the .NET Stopwatch. Only test ours in builds that use it.

Fixes #322.

I modified our Compatibility Stopwatch so that it derives from .NET Stopwatch where it exists. That eliminates any need to test that the public methods pass through. There is only one test in that situation: that the Stopwatch is a System.Diagnostics.Stopwatch.

The tests continue to run in the CF and Silverlight builds. Note that we could use the .NET Stopwatch in the CF builds and once we do so the tests should be eliminated there as well.