CI: core_assertions LoadError in Windows cases

[junaruga]

https://github.com/ruby/drb/actions/runs/32029577375/job/95386338154?pr=54
I got the following CI failure in my fork repository.

https://github.com/junaruga/ruby-drb/actions/runs/32027865148/job/95381019434

Run bundle exec rake
D:/a/ruby-drb/ruby-drb/test/lib/helper.rb:2:in 'Kernel#require': cannot load such file -- core_assertions (LoadError)
	from D:/a/ruby-drb/ruby-drb/test/lib/helper.rb:2:in '<top (required)>'
	from <internal:D:/rubyinstaller-head-x64/lib/ruby/4.1.0+4/rubygems/core_ext/kernel_require.rb>:139:in 'Kernel#require'
rake aborted!
Command failed with status (1)
D:/a/ruby-drb/ruby-drb/vendor/bundle/ruby/4.1.0+4/gems/rake-13.4.2/exe/rake:27:in '<top (required)>'
D:/rubyinstaller-head-x64/bin/bundle:33:in '<main>'
Tasks: TOP => default => test
(See full trace by running task with --trace)
Error: Process completed with exit code 1.

The CI failure was reproduced in this repository too by testing PR #54.

https://github.com/ruby/drb/actions/runs/32029577375/job/95386338154?pr=54

The installed gems by bundle install are below.

• drb (2.2.4)
• power_assert (3.1.0)
• rake (13.4.2)
• test-unit (3.7.8)
• test-unit-ruby-core (1.0.15 f992e1b)

On the previous passed case, the installed gems were below.

https://github.com/ruby/drb/actions/runs/27925208781/job/82626232042#step:3:107

• drb (2.2.4)
• power_assert (3.0.1)
• rake (13.4.2)
• test-unit (3.7.8)
• test-unit-ruby-core (1.0.14)

[junaruga]

I confirmed the following change fixed the Windows CI failures. The line was added by the commit 5f44881.

diff --git a/Gemfile b/Gemfile
index e673971..39549fe 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,4 +4,4 @@ gemspec

 gem "rake"
 gem "test-unit"
-gem "test-unit-ruby-core", git: "https://github.com/ruby/test-unit-ruby-core"
+gem "test-unit-ruby-core"

[junaruga]

This PR #54 is a workaround to pass the CI.