Added an extra GitHub action to run SLMessage tests

.github/workflows/nodejs.yml

@@ -27,3 +27,4 @@ jobs:
     - run: npm ci
     - run: npm run build --if-present
     - run: npm run pretest
+    - run: npm run test-slmessage

package.json

@@ -21,6 +21,7 @@
 	},
 	"scripts": {
 		"test": "mocha test/*.spec.js",
-		"pretest": "eslint --ignore-path .gitignore ."
+		"pretest": "eslint --ignore-path .gitignore .",
+		"test-slmessage": "mocha test/slmessage.spec.js"
 	}
 }