build: avoid compiler warning on 32-bit platform

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>