Change W1ex19 test to better reflect the exercise text

The exercise reads:

implement a function nextPrime that returns the first prime
number that comes after the given number. Use the function isPrime
you just defined.

Since it says "first prime number that comes after the given number", I take that to mean that if the number given is prime, it should not return that number but the next prime number. When the test here is (<n), my implementation breaks for (nextPrime 79). Since 79 is prime, the function returns 83, the next prime number, but the test claims that 79 should be returned.