tests: Use plain close() in mock code

The virportallocatortest.c file is compiled both as a test case
and as a mock library; in the latter case, it can't use
VIR_FORCE_CLOSE() because mock libraries are not linked against
libvirt.

Replace VIR_FORCE_CLOSE() with plain close() to solve the issue.

tests/virportallocatortest.c

@@ -58,7 +58,7 @@ static void init_syms(void)
         return;
 
     host_has_ipv6 = true;
-    VIR_FORCE_CLOSE(fd);
+    close(fd);
 }
 
 int socket(int domain,