Adapt test_mutating_op_like_(add)mul to work with scalar mul as well

[lgoettgens]

I now came across two places (namely thofma/Hecke.jl#1659 for SRows and oscar-system/Oscar.jl#4271 for RootSpaceElems) where we don't have a multiplication between objects, but only scalar multiplication (and corresponding mul! methods).

To make use of the test_mutating_op_* functions, we must have a way to disable multiplications between objects. I achieved this in this PR by adding two kwargs, that state if the left respective right factor of some multiplication is a scalar. If this is the case, we only run the tests where the output is aliased with a non-scalar input. This skips all tests where we would multiply two objects (which errors in the above cases) and the ones where we use a scalar type to save the output in (as these don't really make sense anyway and will fallback to the default impl).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.79%. Comparing base (4013a59) to head (aac5ba3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1892      +/-   ##
==========================================
- Coverage   88.14%   87.79%   -0.35%     
==========================================
  Files         120      120              
  Lines       30300    30276      -24     
==========================================
- Hits        26708    26582     -126     
- Misses       3592     3694     +102     

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[fingolfin]

As always: thank you very much for working on this, I really appreciate it :-)

[lgoettgens]

Yeah, this target type thing seems more natural. Let me try it out and then come back to this.

complete overhaul