Fix minitest + test break on CI for ruby:head

[thdaraujo]

test_i18n_reload sometimes fails with:

Failure: test_faker_i18n(TestI18nLoad)
/home/runner/work/faker/faker/test/test_i18n_reload.rb:40:in 'TestI18nLoad#test_faker_i18n'
     37:     cmd = %( ruby -e "#{code}" )
     38:     output, status = Open3.capture2e(cmd)
     39: 
  => 40:     assert_equal 0, status, output
     41:   end
     42: end

<0> expected but was
<#<Process::Status: pid 2232 exit 1>>

Ref.

It's probably an issue with test dependencies while running on CI, as this test also loads a minitest version that needs to match the one on Gemfile.lock.

I think we could just improve the test by using test-unit without any dependencies. And double-check if we still need this test at all.

[stefannibrasil]

It seems that is failing on ruby head because its minitest version is 5.26.2. We're one version behind, 5.26.1. i like the idea of simply using test-unit for this, I'm going to check if it works.

[stefannibrasil]

The test is still valid, IMO. It was introduced in #2723 to ensure we are not loading i18n twice.