Add tests for off-axis projections

[chrishavlin]

#5080 exposed a lack of tests for off-axis projections, #5081 fixes the bug but does not add tests. So adding this issue to remind myself to add new tests. See this comment for a possible test for SPH data, can do something similar for gridded data.

While adding tests can check the impact of the magnitude of the default depth margin (see this comment).