To avoid corrupting data, Soledad Server checks all user databases
during startup to make sure all of them use the correct schema version.
This was done synchronously, so when there are many databases startup
would take a long time. This commit makes that verification
asynchronous, thus speeding up server startup.

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