Description

Many of the CI runs have intermittent failures and/or intermittent retries. These often happen when tests involving clustered servers are running parallel. Currently, Windows & Ubuntu CI runs on x86-64 systems with two h/t cores. macOS 13 runs on the same. macOS 14 and 15 run on three arm cores.

This PR limits the number of Minitest threads running parallel testing to two (MT_CPU = 2). It will probably slow testing down, but we should have fewer failures and fewer test retries, which saves time.

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• 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.
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• All new and existing tests passed, including Rubocop.