compose test [nfc]: Make trailing whitespace explicit in test cases

The test case 'whitespace around info string' includes a line that
ends with a white space, while this is important for the test case
it can cause two issues:
* It could accidentally get reformatted by code editor
* It is not explicit for developers reading the code

This commit is making the trailing white space be more explicit.

Author: Khader-1

test/model/compose_test.dart

@@ -95,9 +95,10 @@ world
     });
 
     test('whitespace around info string', () {
+      const infoString = ' javascript ';
       checkFenceWrap('''
 ````
-``` javascript 
+```$infoString
 // hello world
 ```
 ````