internal/database: fix flaky test
internal/database: fix flaky test

Reduce the number of concurrent transactions so that there is always a
conflict, but they don't reach the max number of retries.

Change-Id: I15bb6c710473cbddcf070517f0eb907d708a46a7
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/325699
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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