Incorporating test-vectors in the NMT specs

# Problem
EPIC: https://github.com/celestiaorg/celestia-app/issues/1296.

Based on the comments by @liamsi in https://github.com/celestiaorg/nmt/pull/101#pullrequestreview-1300095494

It would be beneficial to include an **appendix** of realistic test vectors in the NMT spec for implementers to use when verifying their implementation. These test vectors should specifically cater to the nmt implementation for Celestia and **match its parameters**.

# Acceptance Criteria

Once there is complete confidence in the implementation, proper test vectors shall be generated and added to the specs.