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

@@ -8,7 +8,7 @@ import pytest
 @pytest.mark.parametrize('single_link', [False, True])
 def test_equivalence_cython_python_version(single_link):
     if not cil.cython_funcs_available:
-        return
+        pytest.skip("Skipped: Cython functions are not available.")
 
     stencil_2d = get_stencil("D2Q9")
     stencil_3d = get_stencil("D3Q19")
@@ -44,7 +44,7 @@ def test_equivalence_cython_python_version(single_link):
 @pytest.mark.parametrize('single_link', [False, True])
 def test_equivalence_cell_idx_list_cython_python_version(single_link):
     if not cil.cython_funcs_available:
-        return
+        pytest.skip("Skipped: Cython functions are not available.")
 
     stencil_2d = get_stencil("D2Q9")
     stencil_3d = get_stencil("D3Q19")