Add test case for `TFCSRadialConfiguration.TF_INSIDE_CS` to `test_stresscl()`

[clmould]

Currently test_stresscl() only tests the i_tf_inside_cs=TFCSRadialConfiguration.TF_OUTSIDE_CS option. The choice of TFCSRadialConfiguration will change how a_cs_poloidal is calculated. A test case should be added to test for i_tf_inside_cs=TFCSRadialConfiguration.TF_INSIDE_CS