A lot of hung test-run workers in CI

[Totktonada]

We see a lot of situations in CI that looks like hung workers. This looks as a problem in test-run or testing infrastructure, not even in tests.

I propose to start debugging from test-run: either run workers under strace or instrument them with color_stdout() prints and perform runs in CI to spot the case. Can we reach some resource limit (say, number of file descriptors) and spin trying to acquire it?

May it be relevant to the recent transition to Python 3?

Example: https://github.com/tarantool/tarantool/runs/2126385328

[Totktonada]

The list of fixed and known problems around hanging workers is below.

• Fix hang on GC in Colorer._write() test-run#275
• test-run worker hangs, when an instance does not enter to the event loop test-run#276