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.cpp

@@ -4456,16 +4456,16 @@ bool ChainstateManager::ProcessNewBlockHeaders(std::span<const CBlockHeader> hea
 
 void ChainstateManager::ReportHeadersPresync(const arith_uint256& work, int64_t height, int64_t timestamp)
 {
-    AssertLockNotHeld(cs_main);
+    AssertLockNotHeld(GetMutex());
     {
-        LOCK(cs_main);
+        LOCK(GetMutex());
         // Don't report headers presync progress if we already have a post-minchainwork header chain.
         // This means we lose reporting for potentially legitimate, but unlikely, deep reorgs, but
         // prevent attackers that spam low-work headers from filling our logs.
         if (m_best_header->nChainWork >= UintToArith256(GetConsensus().nMinimumChainWork)) return;
         // Rate limit headers presync updates to 4 per second, as these are not subject to DoS
         // protection.
-        auto now = std::chrono::steady_clock::now();
+        auto now = MockableSteadyClock::now();
         if (now < m_last_presync_update + std::chrono::milliseconds{250}) return;
         m_last_presync_update = now;
     }