[SDESK-7442] Fix and improve tests

[eos87]

Purpose

To fix the tests that are related to EventsAsyncService.

What has changed

• Added a helper method (clone_with) in EventResourceModel to facilitate the cloning and deep merging of a model instance and a given dict of updates (we can consider moving this base model later on if needed)
• Adjusted EventsAsyncService create and update methods to match the same functionality, as the legacy service uses patch or post method to bypass the hooks. Instead, in the async service, I've abstracted the logic, which is bypassed in the legacy service, to explicitly use it so it is easier to follow and to understand.
• Updated events_tests.py to use new EventsAsyncService
• Improved test case so we don't have to repeat async with self.app.app_context(): all over the tests

What's pending

• There are still references to old events service but we will move them as we move forward
• prod_api tests are still broken but I will take a look at those in SDESK-7456
  SDESK-7442

Thanks for checking!