The as_contract function on the env adds an empty string as the "func" arg to with_test_contract_frame.

Is this needed? If not, the env should remove the "func" arg. If it is, we should add the func argument to as_contract.

It appears the "func" arg impacts the result of require_auth, so it might be useful for people to have when testing custom auth. For an example, see: https://github.com/stellar/rs-soroban-sdk/blob/main/tests/account/src/lib.rs

This was brought up in discussion on this PR: stellar/rs-soroban-env#1628