user32: Add a test showing a problem in GetClassNameA.

dlls/user/tests/class.c

@@ -300,6 +300,8 @@ static void test_instances(void)
     HINSTANCE kernel32 = GetModuleHandleA("kernel32");
     HINSTANCE user32 = GetModuleHandleA("user32");
     HINSTANCE main_module = GetModuleHandleA(NULL);
+    DWORD r;
+    char buffer[0x10];
 
     memset( &cls, 0, sizeof(cls) );
     cls.style         = CS_HREDRAW | CS_VREDRAW;
@@ -411,6 +413,10 @@ static void test_instances(void)
         "Didn't get kernel32 class for null instance\n" );
     DestroyWindow( hwnd2 );
 
+    r = GetClassName( hwnd, buffer, 4 );
+    todo_wine ok( r == 3, "return wrong\n");
+    ok( !strcmp( buffer, "__t"), "name wrong\n");
+
     ok( UnregisterClassA( name, kernel32 ), "Unregister failed for kernel32\n" );
 
     hwnd2 = CreateWindowExA( 0, name, "test_window", 0, 0, 0, 0, 0, 0, 0, NULL, 0 );