Fill out unit tests on TotScatCalculateSelfScatteringTest #34328
Description
While working on #34272 it has been noticed that the unit tests are quite basic and simply check that a) the algorithm runs, b) it returns a workspace containing one spectrum per group.
These could be improved by mocking the child algorithms and checking they're called as expected (eg CalculatePlaczek). In order to do this it's probably the case that all the child algorithms will need to be mocked so that the absence of an output workspace from the previous step in the chain doesn't cause any issues