Add a test for the linear solver SPRAL

[amontoison]

@odow Do you know how we can implement the following code without JuMP?

model = Model(Ipopt.Optimizer)
@variable(model, x)
@objective(model, Min, (x - 2)^2)

# Use the linear solver SPRAL
set_attribute(model, "linear_solver", "spral")
optimize!(model)
@test value(x) == 2.0

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Codecov Report

Patch and project coverage have no change.

Comparison is base (037c159) 94.02% compared to head (25ea437) 94.02%.

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```diff @@ Coverage Diff @@ ## master #380 +/- ## ======================================= Coverage 94.02% 94.02% ======================================= Files 4 4 Lines 921 921 ======================================= Hits 866 866 Misses 55 55 ```

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[chriscoey]

@amontoison try this (no need for JuMP dependency):

    model = Ipopt.Optimizer()
    MOI.set(model, MOI.RawOptimizerAttribute("linear_solver"), "spral")    
    MOI.set(model, MOI.Silent(), true)
    x = MOI.add_variable(model)
    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
    f = (1.0 * x - 2.0) ^ 2
    MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f)
    MOI.optimize!(model)
    @test MOI.get(model, MOI.VariablePrimal(), x) ≈ 2.0

[odow]

Looks like this worked.

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