RCBC-535: Upgrade to Minitest 6

[DemetrisChr]

The only change we need is adding minitest-mock as a dependency (no longer bundled with minitest), which is needed for the active support test cases.

[Copilot]

Pull request overview

Upgrades the development dependency on Minitest to the 6.x series and adds the newly-separate minitest-mock gem (Minitest 6 no longer bundles mock/stub support), which is required by the Active Support–derived test cases that rely on Time.stub and ActiveSupport::Testing::MethodCallAssertions. The PR also re-enables three previously skipped scan concurrency tests.

Changes:

• Pin minitest to ~> 6.0 (previously < 6.0) in the :development group of the Gemfile.
• Add minitest-mock as an explicit development dependency.
• Remove skip("Skipped until CXXCBC-345 is resolved") from the three scan concurrency tests in test/scan_test.rb.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Gemfile	Bumps minitest to ~> 6.0 and adds the standalone minitest-mock gem needed for Time.stub and friends.
test/scan_test.rb	Removes CXXCBC-345 skip guards from the range/prefix/sampling scan concurrency tests, re-enabling them in CI.

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