refactor: Use array initialization for unterminated strings

The previous code is correct and harmless to initialize an array with a
non-terminated character sequence using a string literal.

However, it requires exactly specifying the array size, which can be
cumbersome.

Also, GCC-15 may issue the -Wunterminated-string-initialization warning.
[1]

Fix both issues by using array initialization. This refactoring commit
does not change behavior.

[1] Example warning:

src/modules/schnorrsig/main_impl.h:48:46: error: initializer-string for array of 'unsigned char' is too long [-Werror=unterminated-string-initialization]
   48 | static const unsigned char bip340_algo[13] = "BIP0340/nonce";
      |                                              ^~~~~~~~~~~~~~~

src/testrand_impl.h

@@ -18,7 +18,7 @@
 static uint64_t secp256k1_test_state[4];
 
 SECP256K1_INLINE static void testrand_seed(const unsigned char *seed16) {
-    static const unsigned char PREFIX[19] = "secp256k1 test init";
+    static const unsigned char PREFIX[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};
     unsigned char out32[32];
     secp256k1_sha256 hash;
     int i;