init: Initialize globals with kernel::Context's life

...instead of explicitly calling init::{Set,Unset}Globals.

Cool thing about this is that in both the testing and bitcoin-chainstate
codepaths, we no longer need to explicitly unset globals. The
kernel::Context goes out of scope and the globals are unset
"automatically".

Also construct kernel::Context outside of AppInitSanityChecks()

src/kernel/context.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) 2022 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 <kernel/context.h>
+
+#include <crypto/sha256.h>
+#include <key.h>
+#include <logging.h>
+#include <pubkey.h>
+#include <random.h>
+
+#include <string>
+
+
+namespace kernel {
+
+Context::Context()
+{
+    std::string sha256_algo = SHA256AutoDetect();
+    LogPrintf("Using the '%s' SHA256 implementation\n", sha256_algo);
+    RandomInit();
+    ECC_Start();
+    ecc_verify_handle.reset(new ECCVerifyHandle());
+}
+
+Context::~Context()
+{
+    ecc_verify_handle.reset();
+    ECC_Stop();
+}
+
+} // namespace kernel