Tests that were imported from u1db or created on top of that structure
were leaving temporary directories behind. This could cause problems in
test servers, either by filling the partition or by extrapolating the
maximum amount of files in a directory.

This commit replaces all usages of temporary directories in the old test
structure by pytest tmpdir fixture, which properly cares for removing
temporary directories.

TestSyncEncrypterPool.test_encrypt_doc_and_get_it_back was trying to do
an operation and asserting the number of attempts. This test is about
putting a doc on encrypter pool and getting it encrypted. If we dont
wait for the encryption operation to succeed, then complex
trial-and-error happens, but if we just ask twisted to wait for one
operation before going to the other, this is not needed.

-- Resolves: #8398

test_processing_order aims to check that unordered docs wont be
processed, but if we let the pool start and advance Twisted LoopingCall
clock right before calling the processing method manually, the process
method will run concurrently and cause a race condition issue.

- move tests to root directory
- split tests in different subdirectories
- setup a small package with common test dependencies in /testing/test_soledad
- add tox.ini that will:
  - install the test_soledad package and other test dependencies
  - install soledad common, client, server from the repository
  - run tests contianed in /testing/tests directory using pytest

This commit also removes all oauth code from tests, as we have removed the
u1db dependency (by importing it into the repo and naming it l2db) and don't
neet oauth at all right now.