This is my test with MessageBoxW:

Result should be "TestingString"

Code:
    char *messs = (char*)LockResource(hGlobal3);

    MessageBox(NULL, (LPCTSTR)messs, "bonjour", MB_OK);
    MessageBoxW(NULL, (LPCWSTR)messs, (LPCWSTR)"bonjour", MB_OK);
MessageBoxA only shows the first letter.
MessageBoxW shows the incomplete "Testin"-characters with strange things around it.