Use assert_ractor for separating test processes

Merged: https://github.com/ruby/ruby/pull/4464

test/test_prime.rb

@@ -262,13 +262,12 @@ class TestPrime < Test::Unit::TestCase
       assert_equal 861, (100_000..110_000).count(&:prime?)
     end
 
-=begin
-  # now Ractor should not use in test-all process
     def test_prime_in_ractor
+      assert_ractor(<<~RUBY, require: 'prime')
         # Test usage of private constant...
         assert_equal false, Ractor.new { ((2**13-1) * (2**17-1)).prime? }.take
+      RUBY
     end if defined?(Ractor)
-=end
   end
 
   def test_eratosthenes_works_fine_after_timeout