Makes 'test_invalid_with_letters' test more robust by including 10 digits

This makes "test_invalid_with_letters" for the "phone-number" assignment more robust. 

This prevents phone numbers that have 10 digits mixed with numbers from passing. The use case of a phone number that has more than or less than 10 digits is already covered in the other tests, but the use case where a 10 digit number has additional numbers is not covered.