Test suite no longer stalls a release

Three tests that wait for a delegated sub-thread used a retry loop that only paused between attempts when the underlying read failed. When the read instead returned "nothing yet", the loop retried tens of thousands of times a second, pinned a processor and never finished — taking the whole test run down with it and, on twelve occasions in a single day, blocking the build that ships new versions. The three tests now wait for the sub-thread to appear instead of polling for it, and a new build check refuses any future test that reintroduces the same loop.