test_child_process: Try to fix unstable tests

[ashie]

On CI environment, these tests sometimes observe unexepected 3rd command
invocation due to delayed shutdown:

2022-06-13T23:57:56.4703110Z Failure: test: can execute external command many times, with leading once executed immediately(ChildProcessTest)
2022-06-13T23:57:56.4704700Z /Users/runner/work/fluentd/fluentd/test/plugin_helper/test_child_process.rb:345:in `block (2 levels) in <class:ChildProcessTest>'
2022-06-13T23:57:56.4705540Z      342:       end
2022-06-13T23:57:56.4706110Z      343:       sleep 1.9 # 1sec(second invocation) + 0.9sec
2022-06-13T23:57:56.4706880Z      344:       @d.stop; @d.shutdown; @d.close; @d.terminate
2022-06-13T23:57:56.4707620Z   => 345:       assert_equal 2, ary.size
2022-06-13T23:57:56.4708220Z      346:       assert_equal [], @d.log.out.logs
2022-06-13T23:57:56.4708800Z      347:     end
2022-06-13T23:57:56.4709330Z      348:   end
2022-06-13T23:57:56.4709970Z /Users/runner/hostedtoolcache/Ruby/2.7.6/x64/lib/ruby/2.7.0/timeout.rb:95:in `block in timeout'
2022-06-13T23:57:56.4710800Z /Users/runner/hostedtoolcache/Ruby/2.7.6/x64/lib/ruby/2.7.0/timeout.rb:33:in `block in catch'
2022-06-13T23:57:56.4711540Z /Users/runner/hostedtoolcache/Ruby/2.7.6/x64/lib/ruby/2.7.0/timeout.rb:33:in `catch'
2022-06-13T23:57:56.4712240Z /Users/runner/hostedtoolcache/Ruby/2.7.6/x64/lib/ruby/2.7.0/timeout.rb:33:in `catch'
2022-06-13T23:57:56.4712940Z /Users/runner/hostedtoolcache/Ruby/2.7.6/x64/lib/ruby/2.7.0/timeout.rb:110:in `timeout'
2022-06-13T23:57:56.4713660Z /Users/runner/work/fluentd/fluentd/test/plugin_helper/test_child_process.rb:339:in `block in <class:ChildProcessTest>'
2022-06-13T23:57:56.4714530Z <2> expected but was
2022-06-13T23:57:56.4715340Z <3>

To avoid it, check only 1st & 2nd elapsed time.

Signed-off-by: Takuro Ashie [email protected]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:

Docs Changes:

Release Note: