Use pytestmark in atomic remote tests

The individual `@pytest.mark.remote_data` decorators have been removed
from `atomic` remote tests and are replaced with the `pytestmark` global
variable. This serves as an example other sub-packages can follow.

Author: eerovaher

astroquery/atomic/tests/test_atomic_remote.py

@@ -6,8 +6,9 @@
 
 from ...atomic import AtomicLineList
 
+pytestmark = pytest.mark.remote_data
+
 
-@pytest.mark.remote_data
 def test_default_form_values():
     default_response = AtomicLineList._request(
         method="GET", url=AtomicLineList.FORM_URL,
@@ -28,7 +29,6 @@ def test_default_form_values():
         'wave': u'Angstrom'}
 
 
-@pytest.mark.remote_data
 def test_query_with_default_params():
     table = AtomicLineList.query_object(cache=False)
     assert isinstance(table, Table)
@@ -43,7 +43,6 @@ def test_query_with_default_params():
        1.02916   Zn XXX  E1         1*-6*  1-6 1/2-* 1.33E+12 0.00 - 97174700.00'''.strip()
 
 
-@pytest.mark.remote_data
 def test_query_with_wavelength_params():
     result = AtomicLineList.query_object(
         wavelength_range=(15 * u.nm, 200 * u.Angstrom),
@@ -66,15 +65,13 @@ def test_query_with_wavelength_params():
                             '0.00 -   502481.80']))
 
 
-@pytest.mark.remote_data
 def test_empty_result_set():
     result = AtomicLineList.query_object(wavelength_accuracy=0, cache=False)
     assert isinstance(result, Table)
     assert not result
     assert len(result) == 0
 
 
-@pytest.mark.remote_data
 def test_lower_upper_ranges():
     result = AtomicLineList.query_object(
         lower_level_energy_range=u.Quantity((600 * u.cm**(-1), 1000 * u.cm**(-1))),