xvfb tests fail on Mac OS

[satra]

Summary

nipype tests should skip Xvfb related tests on Mac OS if not available

Actual behavior

                raise RuntimeError(
>                   'Xvfb requires root permissions to run in OSX. Please '
                    'make sure that an X server is listening and set the '
                    'appropriate config on either $DISPLAY or nipype\'s '
                    '"display_variable" config. Valid X servers include '
                    'VNC, XQuartz, or manually started Xvfb.')
E               RuntimeError: Xvfb requires root permissions to run in OSX. Please make sure that an X server is listening and set the appropriate config on either $DISPLAY or nipype's "display_variable" config. Valid X servers include VNC, XQuartz, or manually started Xvfb.

i've tried the following configurations:

1. XQuartz is running.
2. DISPLAY variable is unset

Expected behavior

on platforms where we expect issues, we should ensure that tests are either skipped or users are informed as to what to do.

How to replicate the behavior

Run test on Mac OS

Script/Workflow details

pytest nipype/utils

Platform details:

{'pkg_path': '/software/nipy-repo/nipype/nipype', 'commit_source': 'repository', 'commit_hash': 'fc2c5b4', 'nipype_version': '1.0.0-dev+gfc2c5b4', 'sys_version': '3.6.0 |Continuum Analytics, Inc.| (default, Dec 23 2016, 13:19:00) \n[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]', 'sys_executable': '/software/miniconda3/envs/dev36pype/bin/python', 'sys_platform': 'darwin', 'numpy_version': '1.13.1', 'scipy_version': '0.19.0', 'networkx_version': '1.11', 'nibabel_version': '2.3.0dev', 'traits_version': '4.6.0'}
1.0.0-dev+gfc2c5b4

Execution environment

• My python environment outside container

[oesteban]

Could you confirm that at least this one is passing?:

https://github.com/nipy/nipype/blob/master/nipype/utils/tests/test_config.py#L182-L195

[satra]

yes, that one passes.

[oesteban]

Fixed in #2363.