Updated unit test location
It was changed in https://github.com/HaxeFoundation/haxe/commit/4f5d4a230d5f041dd5f6fe63b9bc79aea9a00b81
fixing gc test again
Mark "void Boot()" as exported

This marks two functions with HXCPP_EXTERN_CLASS_ATTRIBUTES to make it
easier to use hxcpp as a DLL in Windows. Currently it almost is as
simple as just building all of hxcpp with HXCPP_DLL_EXPORT and defining
HXCPP_DLL_IMPORT when using it, except that there are linker errors
involving "void Boot()" and "int __hxcpp_irand(int inMax)" since they
lack the necessary declspec attributes.