Add tests for result count of solution attributes

[odow]

Closes #900

[codecov-io]

Codecov Report

Merging #901 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
- Coverage    95.1%   95.08%   -0.03%     
==========================================
  Files          80       80              
  Lines        8518     8622     +104     
==========================================
+ Hits         8101     8198      +97     
- Misses        417      424       +7

Impacted Files	Coverage Δ
src/Test/UnitTests/solve.jl	100% <100%> (ø)	⬆️
src/Utilities/mockoptimizer.jl	89.85% <100%> (-0.05%)	⬇️
src/attributes.jl	88.6% <100%> (+2.56%)	⬆️

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[blegat]

We should define a custom error to enforce consistency between solvers wrappers, e.g.,

struct ResultIndexBoundsError{AttrType}
    attr::AttrType
    result_count::Int
end
function check_result_index_bounds(model::ModelLike, attr)
    result_count = get(model, ResultCount())
    if !(1 <= attr.result_index <= result_count)
        throw(ResultIndexBoundsError(attr, result_count)
    end
end
Base.showerror(...) = ...

[odow]

Thoughts on making a breaking change by renaming the fields from .N to .result_index?

[blegat]

Thoughts on making a breaking change by renaming the fields from .N to .result_index?

That would break MosekTools. We could instead create a result_index function which could be cleaner than assuming a same field name.

[rschwarz]

Is the idea here that solvers would also call MOI.check_result_index_bounds inside of the relevant get methods?

[blegat]

Yes, exactly, all solvers should do that. They could also call MOI.throw_if_not_valid when it's a getter for an index but we currently have not test for that