tests: Make sure to pass right type to buffer_pack

For some reason, the mingw64 builds were failing on these inputs quite reliably
as the passed value was interpretted as a value larger than UINT32_MAX.

This was not caught before because the value is casted from size_t to uint32_t
implicitly so the MSBs were not affecting the result.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/unittests/torture_threads_buffer.c

@@ -307,7 +307,7 @@ static void *thread_ssh_buffer_add_format(void *threadid)
                          q,
                          s,
                          "rocks",
-                         7,
+                         (size_t)7,
                          "So much",
                          "Fun!");
     assert_int_equal(rc, SSH_OK);