CI: Skip TestIntegrationSingle#test_closed_listener on macOS

In an effort to have green CI more often.

It was introduced in puma#2700

I've seen it fail like this on macos-10.15 and Ruby
- 2.2: https://github.com/puma/puma/runs/4055009489?check_suite_focus=true#step:9:739
- 2.3: https://github.com/puma/puma/runs/4055009508?check_suite_focus=true#step:9:661
- macos-11 2.4: https://github.com/puma/puma/runs/4055009711?check_suite_focus=true#step:9:667

      1) Error:
    TestIntegrationSingle#test_closed_listener:
    EOFError: end of file reached
        /Users/runner/work/puma/puma/test/helpers/integration.rb:160:in `readpartial'
        /Users/runner/work/puma/puma/test/helpers/integration.rb:160:in `block (2 levels) in read_body'
        /Users/runner/work/puma/puma/test/helpers/integration.rb:159:in `loop'
        /Users/runner/work/puma/puma/test/helpers/integration.rb:159:in `block in read_body'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:88:in `block in timeout'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:32:in `block in catch'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:32:in `catch'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:32:in `catch'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:103:in `timeout'
        /Users/runner/work/puma/puma/test/helpers/integration.rb:158:in `read_body'
        /Users/runner/work/puma/puma/test/test_integration_single.rb:187:in `test_closed_listener'
        /Users/runner/work/puma/puma/test/helper.rb:82:in `block (4 levels) in run'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:88:in `block in timeout'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:98:in `call'
        /Users/runner/hostedtoolcache/Ruby/2.2.10/x64/lib/ruby/2.2.0/timeout.rb:98:in `timeout'
        /Users/runner/work/puma/puma/test/helper.rb:80:in `block (3 levels) in run'

And like this on
- macos-10.15 Ruby 2.4: https://github.com/puma/puma/runs/4055009526?check_suite_focus=true#step:9:618
- macos-11 Ruby 2.3: https://github.com/MSP-Greg/puma/runs/3616720219?check_suite_focus=true#step:9:574 (when it was introduced)

      1) Error:
    TestIntegrationSingle#test_closed_listener:
    TimeoutEveryTestCase::TestTookTooLong: execution expired
        /Users/runner/work/puma/puma/test/helpers/integration.rb:124:in `gets'
        /Users/runner/work/puma/puma/test/helpers/integration.rb:124:in `connect'
        /Users/runner/work/puma/puma/test/test_integration_single.rb:187:in `test_closed_listener'
        /Users/runner/work/puma/puma/test/helper.rb:82:in `block (4 levels) in run'
        /Users/runner/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/2.4.0/timeout.rb:103:in `timeout'
        /Users/runner/work/puma/puma/test/helper.rb:80:in `block (3 levels) in run'

test/test_integration_single.rb

@@ -182,6 +182,7 @@ def test_application_logs_are_flushed_on_write
   # listener is closed 'externally' while Puma is in the IO.select statement
   def test_closed_listener
     skip_unless_signal_exist? :TERM
+    skip_if :darwin # been flaky with 2.2, 2.3, 2.5
 
     cli_server "test/rackup/close_listeners.ru", merge_err: true
     read_body connect