tests: only include GameConfig.hpp in source file (no header file)

This reduces compile time from 1m19s to 9s when touching GameConfig.hpp.

(Executed from rwgame/build/tests)
before:
$ make >/dev/null
$ touch ../../rwgame/GameConfig.hpp
$ time make>/dev/null

real	1m18.643s
user	1m13.069s
sys	0m5.266s

after:
$ make >/dev/null
$ touch ../../rwgame/GameConfig.hpp
$ time make>/dev/null

real	0m9.247s
user	0m8.374s
sys	0m0.836s

tests/CMakeLists.txt

@@ -12,6 +12,7 @@ find_package(Boost COMPONENTS filesystem unit_test_framework REQUIRED)
 
 set(TEST_SOURCES
 	"main.cpp"
+        "test_globals.cpp"
 	"test_animation.cpp"
 	"test_archive.cpp"
 	"test_buoyancy.cpp"

tests/test_globals.cpp

@@ -0,0 +1,9 @@
+#include "test_globals.hpp"
+
+#include <GameConfig.hpp>
+
+#if RW_TEST_WITH_DATA
+std::string Global::getGamePath() {
+    return GameConfig("openrw.ini").getGameDataPath();
+}
+#endif

tests/test_globals.hpp

@@ -2,7 +2,6 @@
 #define _TESTGLOBABLS_HPP_
 
 #include <SDL2/SDL.h>
-#include <GameConfig.hpp>
 #include <GameWindow.hpp>
 #include <boost/test/unit_test.hpp>
 #include <core/Logger.hpp>
@@ -105,9 +104,7 @@ public:
     }
 
 #if RW_TEST_WITH_DATA
-    static std::string getGamePath() {
-        return GameConfig("openrw.ini").getGameDataPath();
-    }
+    static std::string getGamePath();
 #endif
 
     static Global& get() {