Migrate tests from ecukes to ert

[davidshepherd7]

Summary

• Migrate all feature tests from ecukes (Cucumber-style .feature files) to ert, the built-in Emacs test framework
• Remove ecukes dependency (Cask, step definitions, support files)
• Clean up test helpers: better assertions, simplified imports, expected failure support via ert
• Drop Emacs 27.2 from CI (auctex install issues)

Test plan

• Run make test to verify all ert tests pass
• Verify CI passes on supported Emacs versions

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