Fix deprecation warning

[dpo]

┌ Warning: allowscalar([true]) is deprecated, use `allowscalar() do end` or `@allowscalar` to denote exactly which operations can use scalar operations.
│   caller = macro expansion at runtests.jl:37 [inlined]
└ @ Core ~/work/KnetNLPModels.jl/KnetNLPModels.jl/test/runtests.jl:37

https://github.com/JuliaSmoothOptimizers/KnetNLPModels.jl/runs/7139382816?check_suite_focus=true#step:6:221

[tmigot]

and two more

┌ Warning: MNIST.traindata() is deprecated, use `MNIST(split=:train)[:]` instead.
└ @ MLDatasets ~/.julia/packages/MLDatasets/EXhDV/src/datasets/vision/mnist.jl:187
┌ Warning: MNIST.testdata() is deprecated, use `MNIST(split=:test)[:]` instead.
└ @ MLDatasets ~/.julia/packages/MLDatasets/EXhDV/src/datasets/vision/mnist.jl:195

https://github.com/JuliaSmoothOptimizers/KnetNLPModels.jl/actions/runs/5532932145/jobs/10095708219#step:6:458

[paraynaud]

#72