add compatibility for minitest 6

[catwomey]

minitest 6.0 shipped breaking API changes that affect ci-queue. This PR adds minitest 6 support while remaining backwards compatible with minitest 5.11+.

Breaking changes in minitest 6

1. Minitest.run_one_method(klass, name) — removed

ci-queue used this to run individual tests in SingleExample#run and LazySingleExample#run. Replaced with klass.new(name).run which works on both minitest 5 and 6.

2. Minitest.__run — renamed to Minitest.run_all_suites

ci-queue prepends a module onto Minitest's singleton class to override __run with its Redis-backed poll loop. minitest 6 renamed this method, so the override no longer fires. We now define both __run and run_all_suites on the prepended module — only the one matching the active minitest version is called;
the other is inert.

3. Plugin loading — now opt-in

minitest 5 auto-discovered plugins via load_plugins inside Minitest.run. minitest 6 removed this. Without it, minitest-reporters' plugin_minitest_reporter_init never fires, so its DelegateReporter is never created. This meant ci-queue's reporters (including BuildStatusRecorder) were set in
Minitest::Reporters.reporters but never wired into the CompositeReporter that Minitest.run actually calls record on.

The consequence: BuildStatusRecorder#record never ran → acknowledge never called → Redis running set never cleared → exhausted? never returned true → the poll loop hung forever.

Fixed by calling Minitest.load_plugins explicitly when ci-queue sets up its reporters, and in run_tests_in_fork (used by bisect).

4. Parallel::Executor thread pool

minitest 6 starts N threads (where N = CPU count) before run_all_suites and joins them afterward via shutdown. ci-queue bypasses the executor entirely — it has its own Redis poll loop — so those threads sit idle on queue.pop and shutdown blocks forever. We now drain the executor after Queue.run
completes.

Backwards compatibility

All changes are guarded by respond_to? checks. No version string comparisons. minitest 5.11+ continues to work unchanged.