fix(curriculum): Improved the tests for Find the length of a String challenge

[RandellDawson]

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I have noticed a few campers get confused regarding the test messages for this challenge. The reason for the confusion is if the user changes the original setup code, but the code would still technically result in the correct solution, the test message would fail but the reason given would not match the reason stated because the test combines the following checks but displays "You should be getting the length of lastName by using .length like this: lastName.length." if either fail.

1. Check that the original setup code is not changed
2. Check the the user gets the length of lastName by using .length like this: lastName.length

See https://www.freecodecamp.org/forum/t/help-w-js-challenge-suspected-bug/274365 for the full discussion.

This PR creates an additional test to make sure the user does not change the original setup code which the tests use.

[ValeraS]

we can delete an after test section.

[RandellDawson]

@ValeraS So we do not need the following for this challenge? That was already there.

### After Test
<div id='js-teardown'>
```js
if(typeof lastNameLength !== "undefined"){(function(){return lastNameLength;})();}
```
</div>

[ValeraS]

I don't see why it's needed.

[moT01]

LGTM! Thanks @RandellDawson! There was no issue for this I take it?