tests: Add Wycheproof ECDH vectors

Adds a test for the ECDH module using the Wycheproof vectors.
We use a python script to convert the JSON-formatted vectors
into C code, in the same spirit as https://github.com/bitcoin-core/secp256k1/pull/1245

Co-authored-by: Sean Andersen <6730974+andozw@users.noreply.github.com>

tools/wycheproof_utils.py

@@ -0,0 +1,12 @@
+# Copyright (c) 2024 Random "Randy" Lattice and Sean Andersen
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://www.opensource.org/licenses/mit-license.php.
+'''
+Utility functions for generating C files for testvectors from the Wycheproof project.
+'''
+
+def to_c_array(x):
+    if x == "":
+        return ""
+    s = ',0x'.join(a + b for a, b in zip(x[::2], x[1::2]))
+    return "0x" + s