End-to-end tests lack coverage of user-initiated data clears

[apasel422]

Both with and without clearing browsing history.

[apasel422]

Two things remain here:

1. attribution/impl/e2e-tests/forget-one-site-conversions.json

   Line 2 in bf7d64a

   "$comment": "TODO: Add coverage for other epochs",

2. Coverage for this code, corresponding to Step 2.