Convert kasp multi-signer tests to pytest

Convert kasp multi-signer tests to pytest

Move the multi-signer test scenarios to the rollover directory and
convert tests to pytest.

- If the KeyProperties set the "legacy" to True, don't set expected
  key times, nor check them. Also, when a matching key is found, set
  key.external to True.
- External keys don't show up in the 'rndc dnssec -status' output so
  skip them in the 'check_dnssecstatus' function. External keys never
  sign RRsets, so also skip those keys in the '_check_signatures'
  function.
- Key properties strings now can set expected key tag ranges, and if
  KeyProperties have tag ranges set, they are checked.