[fix] Correct Refund Amount at TVL Cap

Fixes the issue discovered in https://viewblock.io/thorchain/tx/FF19E1D142A18145A4590412DAEB8A56A9047E0EDA002417920F3126D1612989 where the refund amount had the vault balance added. Also adds a new regression test operator type and the ability to override any constant in mocknet and regression tests with an environment variable for testing conditions like this.