"socket" as one more valid filetype for "file" testing

[fruit]

Checklist

• [ ] make test-all (UNIX) passes. CI will also test this
• [ ] unit and/or integration tests are included (if applicable)
• [x] documentation is changed or added (if applicable)

Description of change

Some files can be sockets. Goss detects already socket files correctly with filetype socket.

[aelsabbahy]

Great catch, thanks!