CI - itermittent fix and RuboCop

[MSP-Greg]

Description

1. test_thread_pool.rb - newer RuboCop fixes - fix RuboCop issue (newer RuboCop)

2. test_cli.rb - fix (?) CI itermittent error. See https://github.com/puma/puma/runs/4073168355?check_suite_focus=true#step:9:602

TestCLI#test_control_clustered:
Errno::EAGAIN: Resource temporarily unavailable
    /Users/runner/work/puma/puma/test/test_cli.rb:28:in `sysread'
    /Users/runner/work/puma/puma/test/test_cli.rb:28:in `wait_booted'

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.