Fix testunit CI hang by requiring test-unit >= 3.7.4

The 3.3 testunit CI job was hanging for 6 hours and timing out.

Problem:
test-unit 3.7.4 changed the TestSuite#run method signature:
- 3.7.3: run(result, run_context: nil, &block)
- 3.7.4+: run(worker_context, &block)

test-queue's TestSuite subclass uses `def run(*); super; end`, which
fails on 3.7.3 when test-unit internally calls run with 2 arguments:

  ArgumentError: wrong number of arguments (given 2, expected 1)

The worker process crashes, but the master process keeps waiting
indefinitely, causing the hang.

Reproduction:

Hangs (with test-unit 3.7.3):
  gem install test-unit -v 3.7.3
  BUNDLE_GEMFILE=gemfiles/testunit.gemfile bundle exec testunit-queue ./test/examples/*_testunit.rb

Works (with test-unit >= 3.7.4):
  gem install test-unit -v 3.7.5
  BUNDLE_GEMFILE=gemfiles/testunit.gemfile bundle exec testunit-queue ./test/examples/*_testunit.rb

Solution:
Pin test-unit to >= 3.7.4 in gemfiles/testunit.gemfile.

Author: S-H-GAMELINKS

gemfiles/testunit.gemfile

@@ -4,6 +4,9 @@
 
 source 'https://rubygems.org'
 
-gem 'test-unit'
+# test-unit 3.7.4 changed `TestSuite#run` method signature.
+# Earlier versions cause ArgumentError in test-queue's TestSuite subclass,
+# crashing workers and causing the master process to hang indefinitely.
+gem 'test-unit', '>= 3.7.4'
 
 gemspec path: '../'