1. Create your NUnit Test project as a Windows Console Application, not as a Class Library.
2. Then make your main Program look like this:

   [TestFixture]
   public class Program
   {
       static void Main(string[] args)
       {
           NUnit.ConsoleRunner.Runner.Main(
               new[]{Assembly.GetExecutingAssembly().Location }
               );
       }
   
       [TestCase("Aa - Bb")]
       public void WhenValidatingForename_should_accept_valid_characters(string validInput)
       {
           var result= new ClassUnderTest().Validate(validInput);
           Assert.IsTrue(result);
       }
   
       [TestCase("X<")]
       public void WhenValidatingForename_should_reject_invalid_characters(string invalidInput)
       {
           var result= new ClassUnderTest().Validate(invalidInput);
           Assert.IsFalse(result);
       }
   }

3. Then, add a reference not only to nunit.framework.dll but also to nunit-console-runner.dll

You now have a self-running executable that runs all your tests, but still behaves in the usual way in a build script, or when running tests inside Visual Studio with TestRunner or Resharper or similar.
NB You may need to check if your build scripts are hard-coded to expect a Test project to be a .dll.