[Tracing] Emit Testcase Creation events

[ViniciustCosta]

Description

Adding emits for TestcaseCreationEvent, where new testcases are created and stored into datastore. This happens during:

• upload_testcase (manual upload)
• corpus_pruning_task
• fuzz_task

Note: For Corpus Pruning, the feature of uploading testcases for crashes found during pruning is commented out (this is being tracked and fixed at #4825). Thus, both the event emission and the unit tests are disabled. I enabled it locally and ran the unit tests, which completed successfully.

As referenced in our tracing design doc, "Testcase Creation" section.
b/394048114

Tests

Unit tests

Added unit tests to guarantee that the events are correctly emitted when a testcase is created in all these contexts. For upload_testcase and corpus_pruning, I leveraged some existing unit tests that already asserted the testcase creation steps. For fuzz_task, it was necessary to create a new test in order to call the testcase creation method and assert the event emission.

Runtime tests

For runtime tests, since it is difficult to actually trigger these code paths locally, I intend to deploy to our dev/staging environments and monitor the events. Evidence of events being emitted in dev:

• Fuzz task (link to logs creating testcase):
  Event:
  Actual testcase created:
  

• Upload testcase event:
  
  Actual testcase created:
  

[vitorguidi]

lgtm, but restrain for merging untill you have seen fuzz task/upload_testcase creation events in a dev deployment