Add script-path validation unit tests for group_key witness scenarios

[dstadulis]

@jharveyb to list issue deliverables

[jharveyb]

In descending priority order:

• Manually generate group witness with a non-empty tapscript root, and verify witness via normal minting flow
• Exercise new VM validation branches
• Exercise group and group anchor verifiers + sortassets
• Coverage for GroupPubKey(), DeriveGroupKey(), and new Has/NeedsGenesisWitnessForGroup helpers
• Exercise new branches in proof.Verify() concerning group key validation
• Validate group key reveal handling in committedProofs()
• Validate store & load of multi-element group witnesses