Refactor TestEventLogStorage in advance of making all storages able t…

…o wipe assets (#24133)

Summary:
Some storages can call .wipe(), others can only call .wipe_asset_key().
Also the tests that involved asset events were doing some funky stuff
with generating events from an entirely separate instance that is no
longer needed since the instance is a fixture and wiped after every test
run.

Test Plan: BK

## Summary & Motivation

## How I Tested These Changes

## Changelog [New | Bug | Docs]

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