[minor] Check for ss before testing

[cbiedl]

Hello, commit

commit 3d6efb1010406ded8f54946de956dc55eb468121
Author: Nikos Mavrogiannopoulos <[email protected]>
Date:   Wed Jan 4 13:41:39 2023 +0100

    tang: allow running standalone (#86)

introduced the usage of the ss command in the test suite. However, if that is not available, the test suite will just time out.

So please implement a check beforehand to bail out early and with an obvious reason. Possibly meson.build could just be enhanced accordingly - I'm not sure whether it's possible to enforce a "dependency" only if later running, else I'd also include a (trivial) patch here.