[wip] test tty on "run tests"

trying to get color output back with new docker-compose.  setting "ansi"
didn't work (it's supposed to be true by default anyways, according to
docs).

DO NOT MERGE!

Change-Id: I8a35c12c910c43bd9b85813b72b2a468ea54b961

.buildkite/pipeline.public-common.yml

@@ -94,6 +94,7 @@ steps:
     plugins:
       - docker-compose#${DOCKER_COMPOSE_PLUGIN_VERSION}:
           run: app
+          tty: true
           env:
           - BUILDKITE_PARALLEL_JOB
           - SCCACHE_BUCKET=readysettech-build-sccache-us-east-2