DAOS-18610 tests: Fix container/boundary timeouts

[liw]

The test employs 30000 threads via
concurrent.futures.ThreadPoolExecutor. Each thread creates a container and sleeps 2 s. In el8’s Python 3.6, this ThreadPoolExecutor allows (os.cpu_count() or 1) * 5 threads to run at the same time, whereas in el9’s Python 3.9, it only allows min(32, (os.cpu_count() or 1) + 4). Log messages produced by my debug PR confirm the latter to be 32, which must be way smaller than the former. If we consider only the time taken by the sleep in every thread, then it takes at least 2 * (30000 / 32) = 1875 s to finish 30000 such threads, already longer than the test timeout of 1200 s.

This patch specifies max_workers explicitly to limit the delay caused by sleeping.

Test-tag: pr test_container_boundary

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'container/boundary.py:BoundaryTest.test_container_boundary - test timeout creating containers'
Status is 'In Progress'
Labels: '2.6.5.p1rc1,2.6.5rc1,2.6.5rc2,2.6.5rc3,2.8.0rc1,ci_2.6_weekly,ci_master_weekly,weekly_test'
https://daosio.atlassian.net/browse/DAOS-18610