build: avoid compiler warning on 32-bit platform
    
    On a 32-bit platform:
    
    virstringtest.c: In function 'mymain':
    virstringtest.c:673: warning: this decimal constant is unsigned only in ISO C90
    
    I already had a comment in the file about the 64-bit counterpart;
    the easiest fix was to make both sites use the standardized macro
    that is guaranteed to work.
    
    * tests/virstringtest.c (mymain): Minimum signed integers are a pain.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>