Create mock_env() alternative that supports non-default address generators (aka. other bech32 prefixes)

Before CosmWasm 2.2, mock_env() returned an invalid address. In 2.2+, the contract address in mock_env() is valid for the default MockApi (bech32 prefix "cosmwasm"). Now it would be great to have some sort of `Env` mocker which returns addresses compatible with other `Api` implementations than `cosmwasm_std::testing::MockApi`.