internal/fetch: use a distinct test database
internal/fetch: use a distinct test database

Tests were failing locally because of concurrent execution with etl
tests.

Change-Id: Ia7ba6d739ed78e0f55747f85e90820d47d0515d4
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/627234
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite