WCS objects set to non-WCS objects for tests

There are a lot of tests for ```nddata``` in which WCS objects areset to ```int```, ```float```, ```ndarray```, ```str``` or ```list``` rather than a WCS object. This means that by including WCS slicing in ```nddata```, which requires the ```wcs``` attribute to be a WCS object, all these tests fail.

Here are the tests I found:

```test_arithmetic_data_wcs```
```test_arithmetics_handle_switches```
```test_arithmetics_meta_func```
```test_arithmetics_mask_func```
```test_slicing_1dmask_ndslice[wcs]```
```test_slicing_all_npndarray_1d```
```test_slicing_all_npndarray_nd```
```test_slicing_all_npndarray_shape_diff```
```test_boolean_slicing```
```test_arithmetic_with_wcs_compare```
```test_arithmetic_with_wcs_compare_fail```
```test_wcs_arithmetic```
```test_wcs_arithmetic_ccd[multiply]```
```test_wcs_arithmetic_ccd[divide]```
```test_wcs_arithmetic_ccd[add]```
```test_wcs_arithmetic_ccd[subtract]```
```test_init_of_subclass_in_convert_unit_to```
```test_pass_nddata```
```test_pass_nddata_and_explicit```
```test_pass_nddata_ignored```
```test_parameter_default_identical_to_explicit_passed_argument```
```test_nddata_init_data_nddata```
```test_nddata_init_data_nddata_subclass```
```test_param_wcs```