Design and implement a software that validates a password applying TDD.
The password will be introduced by the user (as an argument of the method) and should return if the password is valid or not.
A valid password should meet the following requirements:
- Have more than 8 characters
- Contains a capital letter
- Contains a lowercase
- Contains a number
- Contains an underscore
- We want a method that answers if the password is valid or not.
- We don't want to know the reason when the password is invalid (the return value is a boolean)