AuTest: Log and When condition for ATS initialized

[bneradt]

This aims to make the AuTests more reliable by providing a log message
from traffic server that the AutTests can use as a ready condition
before the tests are started. Without this, there were issues with the
tests starting either when the cache was initialized and the ports were
not, or vice versa.

[shinrich]

This appears to be a subset of PR #6931. Only the core infrastructure changes needed to tidy up the autests in PR #6931. It does make sense to land the changes separately in my opinion.

[bneradt]

This appears to be a subset of PR #6931. Only the core infrastructure changes needed to tidy up the autests in PR #6931. It does make sense to land the changes separately in my opinion.

You're right. I'm sorry for the confusion. I initially thought I could break this part that is in this PR as a separate change, but then realized in the process of making this PR that since so many tests explicitly set their own Ready condition on the ATS port being open, that this change would not be effective. That is, it wouldn't help any of the race conditions between the port being open and the cache being ready since so many tests (incorrectly, I think) explicitly set their ready on the port being open and will not use this default logic. I didn't realize that I had actually published this PR (or maybe I forgot to close this one after I made the realization). In any case, I'll close this PR and follow up with Walt's comment offline and see whether it applies to the other PR.