test: Avoid script check worker threads while fuzzing

Threads may execute their function any time after they are spawned, so
coverage could be non-deterministic.

Fix this,

* for the script check worker threads by disabling them while fuzzing.
* for the scheduler thread by waiting for it to fully start and run the
  service queue.

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;