Refactor tests folders

[jaegeral]

We should / will refactor several test related folders in the Timesketch project.

at the moment there is:

• end_to_end_tests
• test_tools
• test_data
• data

In the future we want the following structure:

• tests
  • [ ]test_data - Refactor: Move ./test_data/ to dedicated ./tests/test_data/ directory #3270
  • [ ]test_tools
  • [ ]end_to_end_tests
  • test_events - move the tests_events folder to tests #3185

• Update documentation on how to write tests.