internal/postgres: add tests for inserting non-release versions
internal/postgres: add tests for inserting non-release versions

Test are added to TestInsertModulde to verify that a prerelease or
pseudoversion module is inserted correctly.

Change-Id: I604d3d94cfb7bd16c7869ae45b7c55d7b8d2bf4e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/291649
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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