With the new tools from D11122, converting the remaining tests so they run with both the legacy and new proof formats is trivial.
The hardcoded proof case from abc_rpc_addavalanchenode is removed because it no longer makes sense in this context.
The abc_rpc_avalancheproof test is already covering both formats and needs a separate refactor to take advantage of the new facilities.
The abc_rpc_ecash test uses a legacy hardcoded proof, but since it's not testing proofs at all there is no need to run with the new format.

Depends on D11122.