test: move remaining random test util code from setup_common to random

and drop the util/random dependency on util/setup_common.

This improves code separation and avoids creating a circular dependency if
setup_common needs to call the util/random functions.

src/test/util/random.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <test/util/random.h>
+
+#include <logging.h>
+#include <random.h>
+#include <uint256.h>
+
+#include <cstdlib>
+#include <string>
+
+FastRandomContext g_insecure_rand_ctx;
+
+/** Return the unsigned from the environment var if available, otherwise 0 */
+static uint256 GetUintFromEnv(const std::string& env_name)
+{
+    const char* num = std::getenv(env_name.c_str());
+    if (!num) return {};
+    return uint256S(num);
+}
+
+void Seed(FastRandomContext& ctx)
+{
+    // Should be enough to get the seed once for the process
+    static uint256 seed{};
+    static const std::string RANDOM_CTX_SEED{"RANDOM_CTX_SEED"};
+    if (seed.IsNull()) seed = GetUintFromEnv(RANDOM_CTX_SEED);
+    if (seed.IsNull()) seed = GetRandHash();
+    LogPrintf("%s: Setting random seed for current tests to %s=%s\n", __func__, RANDOM_CTX_SEED, seed.GetHex());
+    ctx = FastRandomContext(seed);
+}