Fix: Boundary Index List Tests now also check `single_link=True`

The error fixed by pystencils!169 (merged) was allowed to pass because the test cases did not check for the single_link=True case.

lbmpy_tests/test_boundary_indexlist_creation.py

@@ -6,10 +6,8 @@ from lbmpy.stencils import get_stencil
 import pytest
 
 @pytest.mark.parametrize('single_link', [False, True])
+@pytest.mark.skipif(not cil.cython_funcs_available, reason='Cython functions are not available')
 def test_equivalence_cython_python_version(single_link):
-    if not cil.cython_funcs_available:
-        pytest.skip("Skipped: Cython functions are not available.")
-
     stencil_2d = get_stencil("D2Q9")
     stencil_3d = get_stencil("D3Q19")
 
@@ -42,10 +40,8 @@ def test_equivalence_cython_python_version(single_link):
         np.testing.assert_equal(result_python_3d, result_cython_3d)
 
 @pytest.mark.parametrize('single_link', [False, True])
+@pytest.mark.skipif(not cil.cython_funcs_available, reason='Cython functions are not available')
 def test_equivalence_cell_idx_list_cython_python_version(single_link):
-    if not cil.cython_funcs_available:
-        pytest.skip("Skipped: Cython functions are not available.")
-
     stencil_2d = get_stencil("D2Q9")
     stencil_3d = get_stencil("D3Q19")