test/fix: refactor unterminated string initializers to brace arrays

- Mirror upstream fix (bitcoin-core/secp256k1 fa67b675)
- Convert tags, test vectors, and constants in -zkp-only modules (ecdsa_adaptor, ecdsa_s2c, musig, bppp, rangeproof, schnorrsig_halfagg)
- Avoid -Wunterminated-string-initialization without changing behavior

src/modules/rangeproof/tests_impl.h

@@ -414,7 +414,7 @@ static void test_single_value_proof(uint64_t val) {
     unsigned char blind[32];
     unsigned char blind_out[32];
     unsigned char nonce[32];
-    const unsigned char message[1] = " "; /* no message will fit into a single-value proof */
+    const unsigned char message[] = { ' ' }; /* no message will fit into a single-value proof */
     unsigned char message_out[sizeof(proof)] = { 0 };
     size_t plen = sizeof(proof);
     uint64_t min_val_out = 0;