ci: remove reference to mock object after test (fluent#5055)

**Which issue(s) this PR fixes**:
Fixes fluent#5054

**What this PR does / why we need it**:
This PR will resolve CI error.

**Docs Changes**:
N/A

**Release Note**:
N/A

---------

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: Watson1978

test/plugin/test_in_object_space.rb

@@ -50,6 +50,10 @@ def test_configure
   end
 
   def test_emit
+    # Force release garbaged objects due to avoid unexpected error by mock objects on `on_timer`
+    # https://github.com/fluent/fluentd/pull/5055
+    GC.start
+
     d = create_driver
 
     d.run(expect_emits: 3)

test/plugin/test_input.rb

@@ -133,5 +133,7 @@ def emit(tag, es)
     assert{ @p.router.object_id != original_router.object_id }
 
     @p.router.emit('mytag.testing', ['for mock'])
+  ensure
+    Fluent::Engine.root_agent.labels['@mytest'] = nil
   end
 end

test/plugin_helper/test_event_emitter.rb

@@ -70,6 +70,8 @@ class Dummy < Fluent::Plugin::TestBase
     d.configure(config_element('ROOT', '', {'@label' => '@mytest'}))
     router = d.event_emitter_router("@mytest")
     assert_equal router_mock, router
+  ensure
+    Fluent::Engine.root_agent.labels['@mytest'] = nil
   end
 
   test 'get root router' do