fix: fixing witness construction of slashing tx

[SebastianElvis]

Also adapts all tests to ensure the witness construction works even if all covenant members provide signatures.

[KonradStaniec]

maybe we can return error early from this function i.e:

if len(covenantSigs) < covenantQuorum {
  return nil, fmt.Errorf("not enough signatures")
}

[SebastianElvis]

Good point! As covenantSigs can contain nil (in order to have the length of covenant committee size), I added the below logic below L303

	// ensure the number of covenant signatures is at least the quorum number
	if numSigs < covenantQuorum {
		return nil, fmt.Errorf("not enough covenant signatures to reach quorum")
	}

does this make sense?