Parallel Test Execution and Refactoring Python Tests

This issue is related to #1499 (closed).

Tasks:

• Update dependencies and conftest.py to enable the usage of pytest-xdist for running tests in parallel.
• Replace the celery-based example data usage with the ExampleData
• Separate integration tests from unit tests and configure pytest-xdist to execute the unit tests in parallel.
• Remove parser tests into plugins
• Remove normalizer tests into plugins (?!)