Fix compatibility with minitest 6

[voxik]

This fixes minitest 5 warnings such as:

$ bundle exec rake test
  shell: /usr/bin/bash -e {0}
/home/runner/.rubies/ruby-head/bin/ruby -w -I"lib:test" -w /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/vendor/bundle/ruby/4.0.0+0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb "test/bcrypt_pnkdf/engine_test.rb" MiniTest::Unit::TestCase is now Minitest::Test. From /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/test/bcrypt_pnkdf/engine_test.rb:58:in '<top (required)>'

... snip ...

as well as minitest 6 error:

$ bundle exec rake test
  shell: /usr/bin/bash -e {0}
/home/runner/.rubies/ruby-head/bin/ruby -w -I"lib:test" -w /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/vendor/bundle/ruby/4.0.0+0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb "test/bcrypt_pnkdf/engine_test.rb" /home/runner/.rubies/ruby-head/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- minitest/unit (LoadError)
	from /home/runner/.rubies/ruby-head/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/test/bcrypt_pnkdf/engine_test.rb:2:in '<top (required)>'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'Kernel.require'
	from /home/runner/.rubies/ruby-head/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/vendor/bundle/ruby/4.0.0+0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:21:in 'block in <main>'
	from /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/vendor/bundle/ruby/4.0.0+0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in 'Array#select'
	from /home/runner/work/bcrypt_pbkdf-ruby/bcrypt_pbkdf-ruby/vendor/bundle/ruby/4.0.0+0/gems/rake-13.3.1/lib/rake/rake_test_loader.rb:6:in '<main>'

Where tha latter is caused by:

minitest/minitest@0bec7a1