CI: Use `Driver::logs` for asserting log messages

[daipom]

Describe the bug

Some tests are asserting log messages as follows.
We should fix these tests.

• Use shutdown: false and manually do d.instance_shutdown
• Take logs from Driver::instance

fluentd/test/plugin/test_in_forward.rb

Lines 186 to 203 in 206b46b

	d.run(shutdown: false, expect_records: 2, timeout: 10) do
	entries = []
	# These entries are skipped
	entries << ['tag1', true, {'a' => 3}] << ['tag2', time, 'invalid record']
	entries += records.map { |tag, _time, record| [tag, _time, record] }
	entries.each {|tag, _time, record|
	# Without ack, logs are sometimes not saved to logs during test.
	send_data packer.write([tag, _time, record]).to_s #, try_to_receive_response: true
	}
	end
	logs = d.instance.log.logs
	assert_equal 2, logs.count { |line| line =~ /got invalid event and drop it/ }
	assert_equal records[0], d.events[0]
	assert_equal records[1], d.events[1]
	d.instance_shutdown

We should not use this approach.

The log messages of the instance will be cleared after the shutdown, but we can still take log messages from Driver::logs.
So we should use Driver::logs and should not use shutdown: false for this purpose.

Note: Driver::logs is the same instance as instance.log.out.logs (= instance.log.logs), but reset will not clear it, just re-assign the new list to DummyLogDevice. So we can use Driver::logs after the shutdown.

• fluentd/lib/fluent/test/driver/base_owner.rb

  Lines 30 to 31 in 206b46b

  	@instance.log = TestLogger.new
  	@logs = @instance.log.out.logs

• fluentd/lib/fluent/test/log.rb

  Lines 32 to 34 in 206b46b

  	def reset
  	@logs = [] if @flush_logs
  	end

To Reproduce

None.

Expected behavior

None.

Your Environment

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Your Configuration

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Your Error Log

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Additional context

The document also says we should use Driver::logs.

https://docs.fluentd.org/plugin-development/plugin-test-code#tests-for-logs