Don't fail if the test run script doesn't exist when using Yarn

npm exits with code 0 in this case, while Yarn exits with code 1.

index.js

@@ -96,7 +96,13 @@ module.exports = (input, opts) => {
 			{
 				title: 'Running tests using Yarn',
 				enabled: () => opts.yarn === true,
-				task: () => exec('yarn', ['test'])
+				task: () => exec('yarn', ['test']).catch(err => {
+					if (err.message.includes('Command "test" not found')) {
+						return;
+					}
+
+					throw err;
+				})
 			}
 		]);
 	}