Add test for ractor race condition on ivar sets

Merged: https://github.com/ruby/ruby/pull/6490

bootstraptest/test_ractor.rb

@@ -1579,4 +1579,43 @@ assert_equal "ok", %q{
   end
 }
 
+assert_equal "ok", %q{
+  module M
+    def foo
+      @foo
+    end
+  end
+
+  class A
+    include M
+
+    def initialize
+      100.times { |i| instance_variable_set(:"@var_#{i}", "bad: #{i}") }
+      @foo = 2
+    end
+  end
+
+  class B
+    include M
+
+    def initialize
+      @foo = 1
+    end
+  end
+
+  Ractor.new do
+    b = B.new
+    100_000.times do
+      raise unless b.foo == 1
+    end
+  end
+
+  a = A.new
+  100_000.times do
+    raise unless a.foo == 2
+  end
+
+  "ok"
+}
+
 end # if !ENV['GITHUB_WORKFLOW']