test_runner: use exit code constants instead of magic number exit code

Author: fossamagna

lib/internal/test_runner/harness.js

@@ -46,7 +46,7 @@ function createProcessEventHandler(eventName, rootTest) {
         `triggered an ${eventName} event.`;
 
       rootTest.diagnostic(msg);
-      process.exitCode = 1;
+      process.exitCode = kGenericUserError;
       return;
     }