Skip nested IRB tests in Ruby Core CI (#675)

Sometimes the subprocess gets stuck in the nested IRB session until timed
out. We don't have enough information to debug it yet, so skip the tests
to unblock CI.

test/irb/test_history.rb

@@ -210,6 +210,14 @@ def with_temp_stdio
   end
 
   class NestedIRBHistoryTest < IntegrationTestCase
+    def setup
+      super
+
+      if ruby_core?
+        omit "This test works only under ruby/irb"
+      end
+    end
+
     def test_history_saving_with_nested_sessions
       write_history ""