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

Soledad's events server might be a pain for tests because if it is not
properly initialized and shut down there might be parts of the server
missing or leftover in the reactor after tests are run. With the recent
changes to the way tests are invocated, there's the need to further
isolate events server startup for benchmarks, otherwise it will conflict
with other tests. This commit moves the events server setup to inside
the pytest_collection_modifyitems() hook, where we can check if the
benchmark subdir was selected and properly start the events server if
needed.

- create payloads before running tests
- use different group names for different amount/sizes
- move legacy index creation to setup function
- limit concurrency on blobmanager to 2 (same as thread pool size)
- add a VACUUM call to observe blobs db size decreasing between tests
- properly cleanup server between blobs test runs
- refactor blobs pipeline so it makes more sense and is easier to read