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.

- 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.