qt: Replace QCoreApplication::quit() with QCoreApplication::exit(0)

Qt 5:
 - no behavior change

Qt 6:
 - this change avoids sending a duplicated `QEvent::Quit`

src/qt/bitcoin.cpp

@@ -308,7 +308,9 @@ void BitcoinApplication::startThread()
 
     /*  communication to and from thread */
     connect(&m_executor.value(), &InitExecutor::initializeResult, this, &BitcoinApplication::initializeResult);
-    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, &QCoreApplication::quit);
+    connect(&m_executor.value(), &InitExecutor::shutdownResult, this, [] {
+        QCoreApplication::exit(0);
+    });
     connect(&m_executor.value(), &InitExecutor::runawayException, this, &BitcoinApplication::handleRunawayException);
     connect(this, &BitcoinApplication::requestedInitialize, &m_executor.value(), &InitExecutor::initialize);
     connect(this, &BitcoinApplication::requestedShutdown, &m_executor.value(), &InitExecutor::shutdown);

src/qt/test/apptests.cpp

@@ -119,6 +119,6 @@ AppTests::HandleCallback::~HandleCallback()
     assert(it != callbacks.end());
     callbacks.erase(it);
     if (callbacks.empty()) {
-        m_app_tests.m_app.quit();
+        m_app_tests.m_app.exit(0);
     }
 }