[refactor] move tests to root of repository

Tests entrypoint was in a testing/ subfolder in the root of the
repository. This was made mainly because we had some common files for
tests and we didn't want to ship them (files in testing/test_soledad,
which is itself a python package. This sometimes causes errors when
loading tests (it seems setuptools is confused with having one python
package in a subdirectory of another).

This commit moves the tests entrypoint to the root of the repository.

Closes: #8952