test: update assertion for de-duplicated error message

Signed-off-by: Seonghyun Hong <s3onghyun.hong@gmail.com>

Author: s3onghyun

test/test_engine.rb

@@ -194,7 +194,7 @@ def multi_workers_ready?; true; end
       e = assert_raise(Fluent::ConfigError) do
         engine.reload_config(new_conf)
       end
-      assert e.message.match?('Unreloadable plugin plugin: dummy_engine_class_var_test')
+      assert e.message.match?('Unreloadable plugin: dummy_engine_class_var_test')
 
       assert_kind_of DummyEngineTestInput, engine.root_agent.inputs[0]
       assert_kind_of DummyEngineTestOutput, engine.root_agent.outputs[0]