Fix few markdownlint warnings (#3825)

test/backend-test/README.md

@@ -1,12 +1,9 @@
-
 # Node.js Test Runner
 
 Documentation: https://nodejs.org/api/test.html
 
 Create a test file in this directory with the name `*.js`.
 
-
-
 ## Template
 
 ```js
@@ -26,7 +23,7 @@ test("Test name", async (t) => {
 });
 ```
 
-## Run 
+## Run
 
 Node.js >=18