[installer] Impossible to run tests after installation

[annoviko]

Introduction
There is a command that is used to run tests:

python3 -m pyclustering.tests

Almost the same behind setup.py test:

python3 setup.py test

Both commands depends on current path. If in case of the second command it is logical to be the same folder, but the first one could be called from every place.

The issue is related to #607.
The issue is blocking: #610.

Acceptance Criteria

• Make it possible to run tests from every current folder using python3 -m pyclustering.tests.