Distribute cxx test tests/cxx-test.cpp to each module tests directory

tests/cxx-test.cpp is removed and splitted into gio/tests/cxx.cpp,
gmodule/tests/cxx.cpp and gobject/tests/cxx.cpp.

Helps issue #1434

tests/cxx-test.cpp

@@ -1,10 +0,0 @@
-#include <glib.h>
-#include <gmodule.h>
-#include <glib-object.h>
-#include <gio/gio.h>
-
-int
-main ()
-{
-  return 0;
-}

tests/meson.build

@@ -51,11 +51,6 @@ tests = {
     'source': 'module-test.c',
     'c_args': ['-DMODULE_TYPE="plugin"'],
   },
-  'cxx-test' : {
-    'source' : 'cxx-test.cpp',
-    'include_directories' : gmoduleinc,
-    'dependencies' : [libgio_dep],
-  },
 }
 
 test_extra_programs = {