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

With this commit all tests on py34 tox environment
are collected.

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.