Use assert_ractor for testing Ractor

[hsbt]

What was the end-user or developer problem that led to this PR?

test_decode_in_ractor and test_encode_in_ractor are not working if we create another Ractor instance. So, It's not working with ruby/ruby repo like ruby/ruby#15043

What is your fix for the problem, implemented in this PR?

Use assert_ractor from test-unit-ruby-core that is assertion collection of ruby-core.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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