TestDomain should unload the test AppDomain

nunit-agent.exe can hang using nunit 2.5.9.10348 if the test assembly creates a thread that is left running on completion of the test case execution. To ensure that the test case code threads are all aborted the test domain class should explicitly unload the test case domain.

NUnit.Util.TestDomain should override the Dispose() method as follows:

public override void Dispose()
{
        base.Dispose();
        Unload();

}