Remove PYTHONPATH hack when requiring test-tools

Test-tools have it's own dependencies like
requests or recently added python-dateutil.

When using hack by injecting test_tools/package/
into to PYTHONPATH env variable, these dependencies
are not installed and it is required to specify
them explicitly in the `deps` section of tox.ini.

This could lead to mismatch between test-tools and
tox.ini versions of dependencies, so this change
makes test-tools installable inside tox instead of
using it's source code directly.