refactor: Use MockableSteadyClock in ReportHeadersPresync

This allows the clock to be mockable in tests. Also, replace cs_main
with GetMutex() while touching this function.

Also, use the ElapseSteady test helper in the p2p_headers_presync fuzz
target to make it more deterministic.

The m_last_presync_update variable is a global that is not reset in
ResetAndInitialize. However, it is only used for logging, so completely
disable it for now.

Without this patch, the tool would report a diff:

cargo run --manifest-path ./contrib/devtools/deterministic-fuzz-coverage/Cargo.toml -- $PWD/bld-cmake/ $PWD/../qa-assets/fuzz_corpora/ p2p_headers_presync 32

...
  4468|     81|        auto now = std::chrono::steady_clock::now();
  4469|     81|        if (now < m_last_presync_update + std::chrono::milliseconds{250}) return;
-                                                                                        ^80
+                                                                                        ^79
...

src/validation.h

@@ -1,5 +1,5 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
-// Copyright (c) 2009-2022 The Bitcoin Core developers
+// Copyright (c) 2009-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -933,7 +933,7 @@ private:
     friend Chainstate;
 
     /** Most recent headers presync progress update, for rate-limiting. */
-    std::chrono::time_point<std::chrono::steady_clock> m_last_presync_update GUARDED_BY(::cs_main) {};
+    MockableSteadyClock::time_point m_last_presync_update GUARDED_BY(GetMutex()){};
 
     std::array<ThresholdConditionCache, VERSIONBITS_NUM_BITS> m_warningcache GUARDED_BY(::cs_main);