Minitest update + code coverage

[spamguy]

In this PR:

• Updates minitest to v6. This shuts up an ugly bundle install message.
• Adds a minitest coverage reporter for core-level tests. Runnable by calling thor test:coverage.
• Adds a pile of new unit tests to bring coverage up to 90.8%.
• Fixes a frozen literal warning thrown during minitest's run.

Note: some unit tests were manually written by me to keep test writing skills fresh, but most were autogenerated by Claude.

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	minitest@​5.27.0 ⏵ 6.0.2	-2
	simplecov@​0.22.0

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