fuzz: [refactor] Use IsValidFlagCombination in signature_checker fuzz target

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

src/test/util/script.cpp

@@ -0,0 +1,13 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <script/interpreter.h>
+#include <test/util/script.h>
+
+bool IsValidFlagCombination(unsigned flags)
+{
+    if (flags & SCRIPT_VERIFY_CLEANSTACK && ~flags & (SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS)) return false;
+    if (flags & SCRIPT_VERIFY_WITNESS && ~flags & SCRIPT_VERIFY_P2SH) return false;
+    return true;
+}