Fix minor bug preventing the use of native pytest command

[drdavella]

This allows the native pytest command to be used to run the test suite. Consequently, it should be easier to run specific test modules and test cases since all native pytest command line options are available.

Am I starting to sound like a broken record? 😉

This isn't included in this changeset, but it might be worth considering using the pytest-runner package instead of providing a custom setuptools command for testing.

[astrofrog]

Huh I've actually been using pytest to run the test suite here for a while (I'm surprised the change here is needed). I don't really use the test` command in setup.py`` so we could remove it.

[drdavella]

Weird. 🤔

Here's what I get before that change (on latest master):

============================= test session starts ==============================
platform darwin -- Python 3.6.4, pytest-3.3.2, py-1.5.2, pluggy-0.6.0
rootdir: /Users/ddavella/glue, inifile: pytest.ini
collected 0 items / 1 errors

==================================== ERRORS ====================================
______________________________ ERROR collecting  _______________________________
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:328: in _getconftestmodules
    return self._path2confmods[path]
E   KeyError: local('/Users/ddavella/glue/glue')

During handling of the above exception, another exception occurred:
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:359: in _importconftest
    return self._conftestpath2mod[conftestpath]
E   KeyError: local('/Users/ddavella/glue/glue/conftest.py')

During handling of the above exception, another exception occurred:
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/py/_path/common.py:377: in visit
    for x in Visitor(fil, rec, ignore, bf, sort).gen(self):
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/py/_path/common.py:418: in gen
    dirs = self.optsort([p for p in entries
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/py/_path/common.py:419: in <listcomp>
    if p.check(dir=1) and (rec is None or rec(p))])
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/main.py:765: in _recurse
    ihook = self.gethookproxy(path)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/main.py:669: in gethookproxy
    my_conftestmodules = pm._getconftestmodules(fspath)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:342: in _getconftestmodules
    mod = self._importconftest(conftestpath)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:378: in _importconftest
    self.consider_conftest(mod)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:401: in consider_conftest
    self.register(conftestmodule, name=conftestmodule.__file__)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:250: in register
    ret = super(PytestPluginManager, self).register(plugin, name)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/pluggy/__init__.py:254: in register
    hook._maybe_apply_history(hookimpl)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/pluggy/__init__.py:650: in _maybe_apply_history
    res = self._hookexec(self, [method], kwargs)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/pluggy/__init__.py:222: in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/pluggy/__init__.py:216: in <lambda>
    firstresult=hook.spec_opts.get('firstresult'),
glue/conftest.py:29: in pytest_configure
    if config.getoption('no_optional_skip'):
../miniconda3/envs/cubeviz-regions/lib/python3.6/site-packages/_pytest/config.py:1225: in getoption
    raise ValueError("no option named %r" % (name,))
E   ValueError: no option named 'no_optional_skip'
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 1.40 seconds ============================