acc: Set PYTHONUNBUFFERED in tests (#2797)

## Why
This ensures that output from Python process is not delayed or stuck,
which is important as we poll output from Python process in some tests,
such as apps/run-local.

Author: denik

acceptance/test.toml

@@ -9,3 +9,6 @@ TimeoutCloud = '120s'
 
 Env.PYTHONDONTWRITEBYTECODE = "1"
 EnvRepl.PYTHONDONTWRITEBYTECODE = false
+
+Env.PYTHONUNBUFFERED = "1"
+EnvRepl.PYTHONUNBUFFERED = false