More modular tests

[Nicholas Krämer]

Would be great to have tests for

• different element types
• static arrays (and non-static arrays)

without manually defining a bunch of matrices.

[Nicholas Krämer]

Would also be great to have some easier-to-access test functions (macros?).
Many tests seem to follow a clear pattern:

• Test that op is (approximately) invariant w.r.t. Matrix(): e.g. @test det(S) \approx det(Matrix(S)), @test size(S) \approx size(Matrix(S)), @test S \ X ≈ Matrix(S) \ X
• Test that op (approximately) commutes with Matrix(): e.g. @test inv(Matrix(S)) == Matrix(inv(S))

This could be realized with a bunch of functions

function test_op_invariant(op, S)
    @test op(S) == op(Matrix(S))
end

function test_op_invariant_approx(op, S, abstol=0.1, reltol=0.1)  # ignore the values
    @test op(S) \approx op(Matrix(S)) abstol=abstol reltol=reltol
end

function test_op_commutes(op, S)
    @test Matrix(op(S)) == op(Matrix(S))
end

# etc.

It seems possible to implement this with a macro; see https://github.com/JuliaLang/julia/blob/bf534986350a991e4a1b29126de0342ffd76205e/stdlib/Test/src/Test.jl#L654-L674 for a starting point.