Fix compatibility with minitest 6

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

Author: voxik

test/bcrypt_pnkdf/engine_test.rb

@@ -1,5 +1,4 @@
 require 'minitest/autorun'
-require 'minitest/unit'
 require 'test_helper'
 
 # bcrypt_pbkdf in ruby
@@ -55,7 +54,7 @@ def bcrypt_pbkdf(password, salt, keylen, rounds)
 end
 
 
-class TestExt < Minitest::Unit::TestCase
+class TestExt < Minitest::Test
   def test_table
     assert_equal table, table.map{ |p,s,l,r| [p,s,l,r,BCryptPbkdf::Engine::__bc_crypt_pbkdf(p,s,l,r).bytes] }
   end