fix(learn): improved tests for validating incrementer function (#39385)

* Update avoid-mutations-and-side-effects-using-functional-programming.english.md

* Update avoid-mutations-and-side-effects-using-functional-programming.english.md

* Update avoid-mutations-and-side-effects-using-functional-programming.english.md

* fix typo in avoid-mutations-and-side-effects-using-functional-programming.english.md

curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/avoid-mutations-and-side-effects-using-functional-programming.english.md

@@ -29,7 +29,15 @@ tests:
   - text: Your function <code>incrementer</code> should not change the value of <code>fixedValue</code> (which is <code>4</code>).
     testString: incrementer(); assert(fixedValue === 4);
   - text: Your <code>incrementer</code> function should return a value that is one larger than the <code>fixedValue</code> value.
-    testString: const newValue = incrementer(); assert(newValue === 5);
+    testString: const __newValue = incrementer(); assert(__newValue === 5);
+  - text: Your <code>incrementer</code> function should return a value based on the global `fixedValue` variable value.
+    testString: |
+      (function() {
+       fixedValue = 10;
+       const newValue = incrementer();
+       assert(fixedValue === 10 && newValue === 11);
+       fixedValue = 4;
+      })();
 
 ```