Stabilize follower tests

[eiri]

PR summary

This PR attempts to stabilize CI for follower's tests.

The list of changes:

• Better debug messages that doesn't show iterator stop as an exception
• Shorter timeouts on tests for faster failing on resource starved CI runners
• Simplify and refactor test runner to do iterations on main loop and run stopper in thread as Timer
• Switch to lazy-loading generator expression in main thread of follower to avoid memory and CPU overhead in tread's buffer

Fixes: s1060

Note: An existing issue is required before opening a PR.

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

The CI occasionally hangs or fails on resource starved containers.

What is the new behavior?

CI should be more stable and fail faster.

Does this PR introduce a breaking change?

• Yes
• No

Other information