refactor(simulation): tidy the test mode code

The test manager is reached through simulation::test() instead of a global,
the result cell is owned by a unique_ptr until the memory table takes it, the
cell value is compared with a margin, strings are built with std::format and
the --test settings moved to apply_test_mode_settings(). The driver's signal
check is folded into UpdateSpeedPosition(), which updates a speed table entry
and reports a signal at stop the head of the train just passed. The test.*
keys are listed with the other settings at the start of the log.

rendering/nullrenderer.cpp

@@ -7,8 +7,9 @@ void null_imgui_renderer::BeginFrame()
 {
     auto &io { ImGui::GetIO() };
     if( false == io.Fonts->IsBuilt() ) {
-        unsigned char *pixels;
-        int width, height;
+        unsigned char *pixels { nullptr };
+        int width { 0 };
+        int height { 0 };
         io.Fonts->GetTexDataAsAlpha8( &pixels, &width, &height );
     }
 }