Fixes #39248 - Migrate tests to Minitest

[jakduch]

What changed

• migrate the Smart Proxy test suite from test-unit to Minitest
• replace ci_reporter with minitest-reporters for Jenkins JUnit output
• load the CI reporter only when Jenkins is running
• update Test::Unit-specific assertions and the Mocha/WebMock integrations

Why

ci_reporter_test_unit is unmaintained and its integration broke when Rake 13.4.2 changed how verbose test options are passed. PR #938 fixed the immediate test invocation failure, but left the obsolete reporting stack in place. This change completes the migration described in the issue and removes that dependency.

Validation

• checked Ruby syntax for all files under test, lib, modules, and tasks
• checked the diff for whitespace errors

Full test execution is left to the upstream Ruby matrix because the local Docker runtime was unavailable.

AI assistance disclosure: This pull request was prepared with assistance from Codex 5.6 Sol High. The changes were reviewed by the author.

[jakduch]

Ruby CI is green now. The three DHCPFreeIps failures came from the test depending on global Proxy::DhcpPlugin.settings initialized by another test file, so I made the setup explicit with load_test_settings.