Migrate the tests themselves back to aiida-core

[sphuber]

The reason is that currently if I need to skip certain tests, because temporarily the export archives are invalid, can only be done by commenting the lines in the test list. Since I do not have access to the functions, I cannot use unittest.skip or something like that. This is dangerous because this way we do not get a message of skipped tests.

Having the test functions themselves in aiida-core is also not a big burden, it is mostly the archives and the code to generate them that is useful to have in this external package.