fix bug #23130 (xp modifier not working), add unit tests

Reverts commit e85c37e161 which was
an error.

I chose to use a scoped_pointer this time around because the
definition of play_controller should not actually depend on the
complete definition of unit_type.

data/test/scenarios/xp_mod.cfg

@@ -0,0 +1,48 @@
+{GENERIC_UNIT_TEST "xp_mod_1" (
+	[event]
+		name = start
+
+		[store_unit]
+			variable=foo
+			[filter]
+				id = alice
+			[/filter]
+		[/store_unit]
+
+		{RETURN {VARIABLE_CONDITIONAL foo[0].max_experience equals 44}}
+	[/event]
+)}
+
+{GENERIC_UNIT_TEST "xp_mod_2" (
+	experience_modifier = 50
+
+	[event]
+		name = start
+
+		[store_unit]
+			variable=foo
+			[filter]
+				id = alice
+			[/filter]
+		[/store_unit]
+
+		{RETURN {VARIABLE_CONDITIONAL foo[0].max_experience equals 22}}
+	[/event]
+)}
+
+{GENERIC_UNIT_TEST "xp_mod_3" (
+	experience_modifier = 75
+
+	[event]
+		name = start
+
+		[store_unit]
+			variable=foo
+			[filter]
+				id = alice
+			[/filter]
+		[/store_unit]
+
+		{RETURN {VARIABLE_CONDITIONAL foo[0].max_experience equals 33}}
+	[/event]
+)}

src/play_controller.cpp

@@ -58,6 +58,7 @@
 #include "tooltips.hpp"
 #include "unit.hpp"
 #include "unit_id.hpp"
+#include "unit_types.hpp"
 #include "whiteboard/manager.hpp"
 #include "wml_exception.hpp"
 
@@ -126,6 +127,7 @@ play_controller::play_controller(const config& level, saved_game& state_of_game,
 	soundsources_manager_(),
 	persist_(),
 	gui_(),
+	xp_mod_(new unit_type::experience_accelerator(level["experience_modifier"].to_int(100))),
 	statistics_context_(level["name"]),
 	undo_stack_(new actions::undo_list(level.child("undo_stack"))),
 	loading_game_(level["playing_team"].empty() == false),

src/play_controller.hpp

@@ -247,6 +247,7 @@ protected:
 
 	//other objects
 	boost::scoped_ptr<game_display> gui_;
+	boost::scoped_ptr<unit_type::experience_accelerator> xp_mod_;
 	const statistics::scenario_context statistics_context_;
 	/// undo_stack_ is never NULL. It is implemented as a pointer so that
 	/// undo_list can be an incomplete type at this point (which reduces the

wml_test_schedule

@@ -84,6 +84,9 @@
 0 event_handlers_in_events_8
 0 filter_vision
 0 has_ally
+0 xp_mod_1
+0 xp_mod_2
+0 xp_mod_3
 #
 # LUA
 #