Enable Aqua.test_ambiguities

[mhauru]

In #2257 I disabled method ambiguity checking, but @devmotion suggested enabling them with

@testset "Aqua" begin
    # Test ambiguities separately without Base and Core
    # Ref: https://github.com/JuliaTesting/Aqua.jl/issues/77
    Aqua.test_all(Turing; ambiguities = false)
    Aqua.test_ambiguities(Turing)
end

We should do that and see if we can fix the 6 ambiguities that it warns about. I'm happy to do this, but off next week, so will take a moment.