Add TEST_QUEUE_SPLIT_GROUPS feature to testunit runner

[arika]

This PR add split groups feature to test-unit runner, and fix error on nested sub_test_cases.
Current version of testunit-queue fails when nested sub_test_cases exists.

# t.rb
require 'test/unit'

class NestedTestCase < Test::Unit::TestCase
  sub_test_case 'sub_test_case 1' do
    test 'test 1' do
      assert true
    end
  end
end

$ TEST_QUEUE_WORKERS=1 testunit-queue test/examples/example_testunit_split.rb
Starting test-queue master (/tmp/test_queue_96944_460.sock)

==> Starting test-queue worker [1] (96960 on mini2023.local) - iterating over /tmp/test_queue_96944_460.sock

Loaded suite specified by test-queue master
Started
Finished in 0.103876 seconds.
-------------------------------------------------------------------------------
1 tests, 1 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
-------------------------------------------------------------------------------
9.63 tests/s, 9.63 assertions/s
/.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner/testunit.rb:33:in `block in failure_count': undefined method `failure_count' for nil:NilClass (NoMethodError)

    (@iterator || @tests).sum { |t| t.instance_variable_get(:@_result).failure_count }
                                                                      ^^^^^^^^^^^^^^
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner/testunit.rb:33:in `sum'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner/testunit.rb:33:in `failure_count'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/iterator.rb:71:in `block in each'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/iterator.rb:29:in `loop'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/iterator.rb:29:in `each'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner/testunit.rb:20:in `run'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:67:in `run_suite'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:45:in `block (2 levels) in run'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:102:in `with_listener'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:41:in `block in run'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:39:in `catch'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunnermediator.rb:39:in `run'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunner.rb:40:in `start_mediator'
	from /.../lib/ruby/gems/3.2.0/gems/test-unit-3.6.1/lib/test/unit/ui/testrunner.rb:25:in `start'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner/testunit.rb:51:in `run_worker'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:290:in `block (2 levels) in spawn_workers'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:285:in `fork'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:285:in `block in spawn_workers'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:282:in `times'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:282:in `spawn_workers'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:227:in `execute_internal'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/lib/test_queue/runner.rb:119:in `execute'
	from /.../lib/ruby/gems/3.2.0/gems/test-queue-0.9.0/exe/testunit-queue:9:in `<top (required)>'
	from /.../bin/testunit-queue:25:in `load'
	from /.../bin/testunit-queue:25:in `<main>'

==> Summary (1 workers in 0.1108s)

    [ 1] 1 tests, 1 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications         1 suites in 0.1088s      (pid 96960 exit 1 )

[koic]

Thanks!

[koic]

I just cut test-queue 0.11.0!
https://github.com/tmm1/test-queue/releases/tag/v0.11.0

[arika]

Thank you!