4.8.1: Tests are a runtime requirement

[dvzrv]

Hi! I've recently updated the package for Arch Linux.
Since we have tooling to detect dependencies of Python packages, we have a policy about removing test directories, which are usually not required during runtime (otherwise our tooling misinterprets e.g. the requirement to have pytest installed, etc.).

Afterwards a user noticed that mayavi does not work without the tests though: https://bugs.archlinux.org/task/77385

https://github.com/enthought/mayavi/blob/05b02f41b141a6bd916faaed23cf82381a4b362e/mayavi/api.py

It would be great to remove the runtime requirement for the tests being installed and to also not install the tests at all.