Merge bitcoin/bitcoin#32158: fuzz: Make partially_downloaded_block more deterministic

fa51310121 contrib: Warn about using libFuzzer for coverage check (MarcoFalke)
fa17cdb191 test: Avoid script check worker threads while fuzzing (MarcoFalke)
fa900bb2dc contrib: Only print fuzz output on failure (MarcoFalke)
fa82fe2c73 contrib: Use -Xdemangler=llvm-cxxfilt in deterministic-*-coverage (MarcoFalke)
fa7e931130 contrib: Add optional parallelism to deterministic-fuzz-coverage (MarcoFalke)

Pull request description:

  This should make the `partially_downloaded_block` fuzz target even more deterministic.

  Follow-up to https://github.com/bitcoin/bitcoin/pull/31841. Tracking issue: https://github.com/bitcoin/bitcoin/issues/29018.

  This bundles several changes:

  * First, speed up the `deterministic-fuzz-coverage` helper by introducing parallelism.
  * Then, a fix to remove spawned test threads or spawn them deterministically. (While testing this, high parallelism and thread contention may be needed)

  ### Testing

  It can be tested via (setting 32 parallel threads):

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

  Locally, on a failure, the output would look like:

  ```diff
   ....
  -  150|      0|            m_worker_threads.emplace_back([this, n]() {
  -  151|      0|                util::ThreadRename(strprintf("scriptch.%i", n));
  +  150|      1|            m_worker_threads.emplace_back([this, n]() {
  +  151|      1|                util::ThreadRename(strprintf("scriptch.%i", n));
   ...
  ```

  This excerpt likely indicates that the script threads were started after the fuzz init function returned.

  Similarly, for the scheduler thread, it would look like:

  ```diff
   ...
     227|      0|        m_node.scheduler = std::make_unique<CScheduler>();
  -  228|      1|        m_node.scheduler->m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { m_node.scheduler->serviceQueue(); });
  +  228|      0|        m_node.scheduler->m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { m_node.scheduler->serviceQueue(); });
     229|      0|        m_node.validation_signals =
   ...
  ```

ACKs for top commit:
  Prabhat1308:
    re-ACK [`fa51310`](fa51310121)
  hodlinator:
    re-ACK fa51310121
  janb84:
    Re-ACK [fa51310](fa51310121)

Tree-SHA512: 1a935eb19da98c7c3810b8bcc5287e5649ffb55bf50ab78c414a424fef8e703839291bb24040a552c49274a4a0292910a00359bdff72fa29a4f53ad36d7a8720

src/test/util/setup_common.cpp

@@ -60,6 +60,7 @@
 #include <walletinitinterface.h>
 
 #include <algorithm>
+#include <future>
 #include <functional>
 #include <stdexcept>
 
@@ -230,6 +231,12 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)
             // Use synchronous task runner while fuzzing to avoid non-determinism
             G_FUZZING ? std::make_unique<ValidationSignals>(std::make_unique<util::ImmediateTaskRunner>()) :
                         std::make_unique<ValidationSignals>(std::make_unique<SerialTaskRunner>(*m_node.scheduler));
+        {
+            // Ensure deterministic coverage by waiting for m_service_thread to be running
+            std::promise<void> promise;
+            m_node.scheduler->scheduleFromNow([&promise] { promise.set_value(); }, 0ms);
+            promise.get_future().wait();
+        }
     }
 
     bilingual_str error{};
@@ -247,7 +254,8 @@ ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)
             .check_block_index = 1,
             .notifications = *m_node.notifications,
             .signals = m_node.validation_signals.get(),
-            .worker_threads_num = 2,
+            // Use no worker threads while fuzzing to avoid non-determinism
+            .worker_threads_num = G_FUZZING ? 0 : 2,
         };
         if (opts.min_validation_cache) {
             chainman_opts.script_execution_cache_bytes = 0;