fuzz: [refactor] Use IsValidFlagCombination in signature_checker fuzz target

Can be reviewed with --color-moved=dimmed-zebra

src/test/util/script.h

@@ -18,4 +18,7 @@ static const CScript P2WSH_OP_TRUE{
            return hash;
        }())};
 
+/** Flags that are not forbidden by an assert in script validation */
+bool IsValidFlagCombination(unsigned flags);
+
 #endif // BITCOIN_TEST_UTIL_SCRIPT_H