internal/postgres: merge getPackagesInUnit tests with GetUnit tests
internal/postgres: merge getPackagesInUnit tests with GetUnit tests

getPackagesInUnit is only used by GetUnit, so the tests are merged with
other tests for GetUnit fieldsets.

TestGetPackagesInUnit is mostly a duplicate of TestGetUnit, so it is
deleted.

TestGetPackagesInUnitBypass is merged into TestGetUnitBypass.

For golang/go#39629

Change-Id: If2801d27ded68b94bfc7b7351d5763d96b8f54c1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/252737
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