[agent] Streaming failed test logs to Buildkite (#1531)

Refactor the BEP json file watcher so that:
- the code is more readable and easier to add new behaviour.
- the parse loop is more reliable.

Also, instead of uploading test logs of all failed tests at once in the
end of the test, it now uploads the test log of failed test immediately
as long as the test action is finished.

Test run: https://buildkite.com/bazel-testing/bazel-bazel/builds/8492

Author: coeuvre