refactor: move SignSignature helpers to test utils

These helpers haven't been used in production code since segwit was
merged more than eight years ago (see commit 605e8473, PR #8149),
so it seems appropriate to move them to the test utils module.

Can be reviewed via `--color-moved=dimmed-zebra`.

src/test/multisig_tests.cpp

@@ -10,6 +10,7 @@
 #include <script/sign.h>
 #include <script/signingprovider.h>
 #include <test/util/setup_common.h>
+#include <test/util/transaction_utils.h>
 #include <tinyformat.h>
 #include <uint256.h>