Allow remote workers to append a message to their connect messages

I'm finding myself doing a lot of leg work to get to remote worker console output. This lets remote workers append a message to the output from the master so that I can make that into a link.

```
TEST_QUEUE_SLAVE_MESSAGE="message with spaces" TEST_QUEUE_RELAY_TOKEN=123 ./test-multi.sh 
```

results in:
```
*** 4 workers connected from fried.shrimp.local after 0.130844s message with spaces
```

One can also just set `@slave_message` in `Runner#prepare`.
message could contain newlines which would be bad