Fix small typo/misleading var name in wrapper output dims calculator test

The calculation of the output dims is being tested in this particular test called `test_generic_calculate_output_dims`.

During the test, the `calculate_output_dims()` method on a query object is patched: https://github.com/DiamondLightSource/httomo/blob/f7f4d96508b5e757a42f22bc10e93bec319854a9/tests/method_wrappers/test_generic.py#L600-L602

However, the name of the variable `memcalc_mock` suggests that the method on the query object being patched is the memory calculator method (ie, `calculate_max_slices()`), which isn't the case.

This variable name is likely a typo, but is misleading, so should be changed to something like `output_dims_calc_mock` to avoid confusion.