Add z_getnewaddress test
Supercedes #3749. Closes#3687.
It takes the commit from @dagurval where a new test case
`rpc_wallet_z_getnewaddress` is added and it adds a check for too many
arguments.
It uses `CheckRPCThrows()` to test the error message of invalid argument.
However, for too many arguments the full help message is displayed, so I
am just testing there is an runtime exception.