De-duplicate add_coin methods to a test util helper

ci/test/06_script_b.sh

@@ -60,6 +60,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/rpc/signmessage.cpp"\
           " src/test/fuzz/txorphan.cpp"\
           " src/test/fuzz/util/"\
+          " src/test/util/coins.cpp"\
           " src/uint256.cpp"\
           " src/util/bip32.cpp"\
           " src/util/bytevectorhash.cpp"\