tests: mark main()'s "argc" as unused

Otherwise gcc emits warnings such as

> test-spawn.c: In function ‘main’:
> test-spawn.c:30:11: warning: unused parameter ‘argc’ [-Wunused-parameter]
>    30 | main (int argc, char *argv[])
>       |       ~~~~^~~~

when "-Wextra" is combined with "-Wall", and "-Wextra" is useful.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>