Update async processor tests

[rafal-ch]

Closes #2449
Or at least, tries to :)

Description

Summary of changes, each in separate commit for clarity. The change that attempts to fix the actual flakiness reported in the issue in pt. 2 below:

1. Modify one_spawn_single_tasks_works() to not use sleep
2. Updates assertion in one_spawn_single_tasks_works__thread_id_is_different_than_main()
   • The original test expected the exact number of worker threads to be spawned (assert_eq!(unique_thread_ids.len(), number_of_threads);), however, the AsyncProcessor can potentially reuse threads if the tasks finish quick enough.
   • One way to reduce the flakiness would be to bump the sleep, but I think the more clear way is to ensure that some (but not too many) worker threads have been spawned
     • Especially because the aim of the test is not to ensure the number of worker threads, but to check that main thread id was not used
3. Reduces the arbitrary number of tasks in executes_10_tasks_for_10_seconds_with_one_thread, just to make the test execute faster
4. Add some clarifying comments
5. Slightly update and rename the executes_10_tasks_for_2_seconds_with_10_thread() and executes_10_tasks_for_2_seconds_with_1_thread() - to use proper values and better reflect the idea behind the test

Before requesting review

• I have reviewed the code myself

[AurelienFT]

Thanks a lot for taking the time to (try to) stabilize this :)