Tracing: Use qt_add_qml_module for tst_flamegraphview

Use the CMake-based qml API to create the
"QtCreator.TstTracingFlameGraphView" module.
This uses qt_add_qml_module, which was introduced with Qt 6.2.

For Qt 6.1 and below, everything compiles and runs like before this
change.

Change-Id: I7b1d7109c5fcff55c7be4b431f21281a63d13332
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/tracing/CMakeLists.txt

@@ -2,6 +2,8 @@ if (WITH_TESTS)
   set(TEST_SOURCES
     runscenegraphtest.cpp runscenegraphtest.h
   )
+else()
+  set(TEST_SOURCES "")
 endif()
 
 set(TRACING_CPP_SOURCES