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.

simulation/simulation.cpp

@@ -111,7 +111,7 @@ state_manager::init_scripting_interface() {
         simulation::Region->insert( memorycell );
     }
     // the cell a test scenery reports its result to
-    simulation::Test.init_result_cell();
+    simulation::test().init_result_cell();
 }
 
 // legacy method, calculates changes in simulation state over specified time