scheduler: don't rely on boost interrupt on shutdown

Calling interrupt_all() will immediately stop the scheduler, so it's
safe to invoke stop() beforehand, and this removes the reliance on boost
to interrupt serviceQueue().

src/test/util/setup_common.cpp

@@ -140,6 +140,7 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
 
 TestingSetup::~TestingSetup()
 {
+    if (m_node.scheduler) m_node.scheduler->stop();
     threadGroup.interrupt_all();
     threadGroup.join_all();
     GetMainSignals().FlushBackgroundCallbacks();