pythonPackages.mahotas: skip `test_moments.test_normalize` and...

pythonPackages.mahotas: skip `test_moments.test_normalize` and `test_texture.test_haralick3d` testcase (#46793)

As stated in #46368, this package seems to have issues with impure tests
(reported in https://github.com/luispedro/mahotas/issues/97).

Unfortunately the `release-18.09` job on Hydra fails at the attempt to
build this package since `test_moments.test_normalize1` and
`test_texture.test_haralick3d` breaks. Until
the root cause is identified, we skip the disabled tests to ensure that
the resulting package is not entirely broken (which can't be confirmed
with `doCheck = false`).

See https://hydra.nixos.org/job/nixos/release-18.09/nixpkgs.python27Packages.mahotas.x86_64-linux
See https://hydra.nixos.org/job/nixos/release-18.09/nixpkgs.python36Packages.mahotas.x86_64-linux
See https://logs.nix.ci/?key=nixos/nixpkgs.46793&attempt_id=b85a638d-dcb0-41d3-ab2a-9616a3bb0175

/cc @xeji @luispedro