Since the future git tag variant uses a future FCV as latest, the last-continuous FCV in that variant is the current latest FCV on master. For instance, latest is currently 6.0, which means in the future git tag variant, last-continuous is 6.0, which means last-lts is 5.0. As a result, unsupportedOldFCV in set_feature_compatibility_version.js is actually calculated as 5.0, which means the setFCV command will succeed when we expect it to fail. We can resolve this by decrementing unsupportedOldFCV by 2 instead of 1. Alternatively, we can disable the test in the future git tag variant, but I don't feel that that is necessary if this will allow us to keep testing setFCV.