Added ability to only run tests in selected scripts in editor

I renamed "Assets/Tests/Run" to "Assets/Tests/Run All" and added an additional "Assets/Tests/Run Selected" that will look for test suites in the selected items and run them.
Note that Unity's MonoScript.GetClass returns null if for any reason the class in a script's fully qualified name doesn't match the filename. This means test suites in a namespace won't be able to be run by "Run Selected", but a warning is logged to the console in this case.