test: add signal isolation class and methods

[mariajgrimaldi]

Description:
This PR adds classes and methods that allow developers to enable/disable events during testing. This implementation is a result of this discussion

Testing instructions:

1. make lms-shell
2. pip install git+https://github.com/eduNEXT/openedx-events@MJG/isolate_signal_sending#egg=openedx-events==0.5.0_alpha
3. Go to openedx/core/djangoapps/user_api/tests/test_views.py L31
4. In UserAPITestCase add the mixin OpenEdxEventsTestCase
5. Execute tests, for example: pytest openedx/core/djangoapps/user_authn/views/tests/test_register.py::RegistrationValidationViewTests
6. Manually check that the signal is not sent

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant
solutions, hacks, quick-and-dirty implementations, concerns about
migrations, etc.

[felipemontoya]

Once QA passes and the OpenEdxEventsTestCase docstring is complete I think we are ready to merge this.