Backport(v1.16): ci: remove reference to mock object after test (#5055)

[daipom]

Which issue(s) this PR fixes:

• Backport ci: remove reference to mock object after test #5055

Fixes #5054

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

Docs Changes:
N/A

Release Note:
N/A

[daipom]

This should fix ObjectSpaceInputTest, but it fails...

[Watson1978]

On v1.16 branch, seems it still causes #5054 with test_in_object_space.rb and test_out_forward.rb combination.

$ bundle exec ruby -I"lib:test" -Eascii-8bit:ascii-8bit ~/.rbenv/versions/3.3.9/lib/ruby/gems/3.3.0/gems/rake-13.3.0/lib/rake/rake_test_loader.rb \
    "test/plugin/test_in_object_space.rb" "test/plugin/test_out_forward.rb" -v          
/home/watson/src/fluentd/lib/fluent/compat/parser.rb:28: warning: csv was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add csv to your Gemfile or gemspec to silence this warning.
Loaded suite /home/watson/.rbenv/versions/3.3.9/lib/ruby/gems/3.3.0/gems/rake-13.3.0/lib/rake/rake_test_loader
Started
ForwardOutputTest: 
  test: Create new connection per send_data:                                                                                                                                                                                                                            .: (0.232909)
  test: Node with security is thread-safe on multi threads:                                                                                                                                                                                                             .: (0.822002)

...(snip)...
  
ObjectSpaceInputTest: 
  test_configure:                                                                                                                                                                                                                                                       .: (0.003062)
  test_emit:                                                                                                                                                                                                                                                            F
=============================================================================================================================================================================================================================================================================================
Failure: test_emit(ObjectSpaceInputTest):
      assert{ emits.length > 0 }
              |     |      |
              |     |      false
              |     0
              []
/home/watson/src/fluentd/test/plugin/test_in_object_space.rb:62:in `test_emit'
     59:     d.run(expect_emits: 3)
     60: 
     61:     emits = d.events
  => 62:     assert{ emits.length > 0 }
     63: 
     64:     emits.each { |tag, time, record|
     65:       assert_equal d.instance.tag, tag
=============================================================================================================================================================================================================================================================================================
: (300.307336)

Finished in 342.326487655 seconds.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
62 tests, 136 assertions, 1 failures, 0 errors, 0 pendings, 3 omissions, 0 notifications
98.3051% passed
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.18 tests/s, 0.40 assertions/s

[Watson1978]

Seems the error is caused by

fluentd/test/plugin/test_out_forward.rb

Line 347 in 57d3738

mock = flexmock($log)

On master branch, flexmock($log) was removed by #4896

[daipom]

Thanks for fixing it!