Disable failing DNS tests on macOS

[stephentoub]

https://github.com/dotnet/corefx/issues/26789
cc: @karelz, @wfurt

[stephentoub]

@dotnet/dnceng, this fireball appears to be an infrastructure issue:
Windows.81.Amd64.Open:Debug-x64
https://mc.dot.net/#/user/stephentoub/pr~2Fjenkins~2Fdotnet~2Fcorefx~2Fmaster~2F/test~2Ffunctional~2Fcli~2F/41d0566c452c11b3e56f90d5c8d04b5d74101b96/workItem/System.Threading.Channels.Tests/wilogs

Executed on wuw81o7sj000D9V
2018-02-02 15:15:20,711: INFO: scriptrunner(33): _main: BuildTools Helix Script Runner v0.1 starting
2018-02-02 15:15:20,711: INFO: helix_test_execution(24): __init__: Copying execution payload files from C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Work\b02058d8-e82b-4dd3-a295-8149c78031be\Unzip to C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Work\b02058d8-e82b-4dd3-a295-8149c78031be\Exec\execution
2018-02-02 15:15:20,711: INFO: proc(23): run_and_log_output: Running: C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Work\b02058d8-e82b-4dd3-a295-8149c78031be\Unzip\RunTests.cmd C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Payload
2018-02-02 15:15:20,711: INFO: proc(26): run_and_log_output: CWD: C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Work\b02058d8-e82b-4dd3-a295-8149c78031be\Unzip
2018-02-02 15:15:20,711: WARNING: proc(37): run_and_log_output: Could not find C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Work\b02058d8-e82b-4dd3-a295-8149c78031be\Unzip\RunTests.cmd on disk.  Execution may fail.
2018-02-02 15:15:20,727: ERROR: logs(87): _log_uncaught_exception: Unhandled error: [Error 267] The directory name is invalid
['  File "C:\\dotnetbuild\\work\\46c8755a-913c-4e41-8b04-aba32589ce25\\Payload\\RunnerScripts\\scriptrunner\\scriptrunner.py", line 101, in <module>\n    sys.exit(main())\n', '  File "C:\\dotnetbuild\\work\\46c8755a-913c-4e41-8b04-aba32589ce25\\Payload\\RunnerScripts\\scriptrunner\\scriptrunner.py", line 97, in main\n    return command_main(_main, [\'script=\', \'args=\'], args)\n', '  File "C:\\dotnetbuild\\scripts\\helix\\cmdline.py", line 48, in command_main\n    return main_method(settings, optlist, args)\n', '  File "C:\\dotnetbuild\\work\\46c8755a-913c-4e41-8b04-aba32589ce25\\Payload\\RunnerScripts\\scriptrunner\\scriptrunner.py", line 47, in _main\n    env=None\n', '  File "C:\\dotnetbuild\\scripts\\helix\\proc.py", line 46, in run_and_log_output\n    shell=False)\n', '  File "C:\\Python\\lib\\subprocess.py", line 710, in __init__\n    errread, errwrite)\n', '  File "C:\\Python\\lib\\subprocess.py", line 958, in _execute_child\n    startupinfo)\n']
2018-02-02 15:15:20,727: INFO: azure_storage(205): _upload: Uploading single blob: 'd136d08cd458402e8a12e54ed3cac6ee.log'
2018-02-02 15:15:20,773: INFO: saferequests(90): request_with_retry: Response complete with status code '201'
2018-02-02 15:15:20,773: INFO: event(43): send: Sending event type Logs
2018-02-02 15:15:20,914: INFO: saferequests(90): request_with_retry: Response complete with status code '201'
Traceback (most recent call last):
  File "C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Payload\RunnerScripts\scriptrunner\scriptrunner.py", line 101, in <module>
    sys.exit(main())
  File "C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Payload\RunnerScripts\scriptrunner\scriptrunner.py", line 97, in main
    return command_main(_main, ['script=', 'args='], args)
  File "C:\dotnetbuild\scripts\helix\cmdline.py", line 48, in command_main
    return main_method(settings, optlist, args)
  File "C:\dotnetbuild\work\46c8755a-913c-4e41-8b04-aba32589ce25\Payload\RunnerScripts\scriptrunner\scriptrunner.py", line 47, in _main
    env=None
  File "C:\dotnetbuild\scripts\helix\proc.py", line 46, in run_and_log_output
    shell=False)
  File "C:\Python\lib\subprocess.py", line 710, in __init__
    errread, errwrite)
  File "C:\Python\lib\subprocess.py", line 958, in _execute_child
    startupinfo)
WindowsError: [Error 267] The directory name is invalid
'module' object has no attribute '__file__'

[MattGal]

@stephentoub this is https://github.com/dotnet/core-eng/issues/2566, I'm going to mark it critical and fix it now. The idea was to handle transient eventhub network failure better but it ended up fighting auto-cleanup too much.