Tests/CompileFeatures: Fix CMP0128OldSameStandard with default OFF

With CMP0128 OLD compiler extensions are enabled by default regardless of the
compiler's default.
Fix the test to always check for the extension flag as it was intended to.

Fixes: 4a0485be7f4ab06201c478f5a46111ab1e8e773e (cmStandardLevelResolver: Avoid
unnecessary flags, fix unset level logic, 2021-04-29)