internal/postgres: truncate search_documents in ResetTestDB
internal/postgres: truncate search_documents in ResetTestDB

Truncate search_documents in ResetTestDB. In the next CL, we will be
dropping the search_documents.packages FK, so the deletes will no longer
cascade.

For golang/go#39629

Change-Id: I76529ad345e8829fdd327ebcdc8b59f0b3d68875
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/262774
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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