tests: Move validation tests to the haskell test suite

[croyzor]

Here's how tests appear when the validator is not installed:

      execution
        test_pba:                               OK
        validate(test_pba):                     SKIPPED
          hugr_validator not installed

they are then treated as failures in the test suite summary, unless the option --ignore-validation is passed. Then, they look the same but aren't marked as failures (and "hugr_validator not installed" is not in red)