Move JRuby's Ant integration specs here as minitest tests - #6

JRuby tested this gem with RSpec specs under spec/java_integration/ant/, which made Ant a requirement for JRuby's own test suite. As discussed in jruby/jruby#9761, this moves those specs here so JRuby can drop them, and adds CI.

What's here

• The 5 JRuby spec files are ported to minitest, one test file each:
  • ant_spec.rb → the new classes in test/test_ant.rb
  • project_spec.rb → test/test_project.rb
  • rake_spec.rb → test/test_rake.rb
  • target_spec.rb → test/test_target.rb
  • task_spec.rb → test/test_task.rb
• The existing test/test_ant.rb test is converted from test-unit to minitest. test-unit was never declared as a dependency, so bundle exec rake test couldn't load it.
• test/ant_test_helper.rb replaces JRuby's ant_spec_helper.rb. The RSpec have_structure / have_configured_structure matchers become assert_structure / assert_configured_structure, using the same matching code.
• Dev dependencies: minitest and minitest-mock. Minitest 6, which ships with Ruby 4.1, moved Object#stub into the separate minitest-mock gem. The Ant.load tests use it in place of RSpec's allow/expect on File.exist?, and still assert that JAVA_HOME, tools.jar and classes.zip were all checked.
• The bundler dev dependency is relaxed from ~> 2.1 to >= 2.1. With current Bundler (4.x, as shipped with JRuby) bundle install failed outright.
• .github/workflows/test.yml runs bundle exec rake test on jruby and jruby-head with Java 21 and Ant.

Differences from the JRuby specs

• The jarjar examples are dropped, as agreed in review. They defined a third-party Ant task with taskdef, which needed jarjar 1.1 on disk. The jar examples still cover structure and configured-structure matching, and the tests need no downloads.
• target_spec's "does not support ant for calling other buildfiles" example is replaced. It built a = example_ant ... but then ran ant.execute_target(:bar) on the global ant, so it passed only because the global project had no :bar target. Ant's <ant> task is in fact supported, so the new test_supports_ant_for_calling_other_buildfiles writes a temporary buildfile, calls it with <ant> from a Ruby-defined target, and asserts its target ran. The antcall example next to it is accurate: antcall fails because it tries to re-read the project's build.xml, which a Ruby-defined project doesn't have.
• rake_spec.rb's top-level def import(*args); java_import(*args); end is not ported. Nothing here calls it, and in JRuby it leaked into other specs that loaded in the same process.
• Two mutable string literals (message = "") are now String.new, because rake test runs with -w and they warn about frozen string literals.

Testing

Locally with JRuby master (Java 21, Ant 1.10.18): bundle exec rake test → 30 runs, 65 assertions, 0 failures, 0 skips, no warnings. That's the 31 JRuby examples minus the 2 jarjar ones, plus the original ant_import test.

CI on my fork: rake test passed on jruby and jruby-head.

Co-authored with Claude Code.