Document nargo test

[Savio-Sou]

• feat(nargo): Add nargo test command to run all unit tests noir#728
• fix(nargo): nargo test now only runs test functions defined in the current module noir#805
• Example: https://github.com/TomAFrench/keccak256-noir/blob/master/lib/src/permutations.nr#L53

[critesjosh]

reference https://discord.com/channels/563037431604183070/1084249281797558353/1087526508454760578