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

Introduction of local services authentication added a configuration file
containing the auth tokens for each service. There were different names
for that file, and this commit standardizes all of them to the same
value: /etc/soledad/services.tokens

Soledad Server was previously using something in /srv to store blobs in
the server side. Debian/lintian doesn't like that at all, so we are
changing to /var/lib/soledad/blobs.

Closes: #8948

AES can generate base64 valid strings, making this test flaky. Try to
decrypt using armor=False instead.

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.

Use subdir path to select/deselect benchmark tests.

Because of the way pytest and pytest benchamrk select/deselect tests, it
makes more sense for this test suite to use a --subdir option to select
only a subset of tests and a blacklist of subdirectories that should not
be run unless explicitelly asked to.

This commit adds a --subdir option, that will deselect all tests no in
./tests/<subdir> when the option is passed to pytest. Also, a blacklist
is added so, unless explicitelly added as a --subdir, tests in
blacklisted subdirectories will not be run.

The reasons for this modification are:

- pytest-benchmarks selects/deselects tests based on the type of their
  fixtures. This means that fixtures have to be instantiated previous to
  test selection/deselection, something that may cause side effects that
  we want to avoid.

- the usual test suite is run for all benchmarks, while some tests
  should only be run in specific situations (as is the case of
  benchmarks and responsiveness tests). It is saner to implement subdir
  selection than marking all tests with pytest marks and using them to
  select/deselect.