move aiida-core tests to pytest

[ltalirz]

We are already recommending pytest for plugins because of generally increased usability & the potential for code reuse via fixtures.
At some point we should move aiida-core tests to pytest as well.

• move all tests outside the aiida folder and inside a top-level tests folder with the same folder hierarchy. This will significantly reduce the size of the aiida package on PyPI. We can think whether it is then still necessary to rebuild the test hierarchy of verdi devel tests or whether it might be just simpler to do pytest tests/the/tests/I/want/to/run
• get rid of the need for creating a "test profile" by defaulting to the dynamically created postgres cluster provided by pgtest. Potentially give users who don't want this overhead the option to stay with their system postgres cluster by fixing faster plugin tests #2648

as discussed with @sphuber