Refactor: cts: Move validation code into a new python module.

This is also going to be useful for running command line tool regression
tests, so split the code out into a separate function and make it more
generic.

• Raise FileNotFoundError if the validator is not found so we can tell that error apart from validation errors.

• Add the xmllint return code to XmlValidationError so we can access it later if needed.

• Return the xmllint output instead of printing it so the caller can decide what to do with it.

• Use configure.ac to find the path to xmllint.