internal/frontend: add test cases for goldmark readme processing
internal/frontend: add test cases for goldmark readme processing

Adds additional test cases and those from original blackfriday set.
Refactors tests to use sample unit instead of module info.

Change-Id: I98668b9bb28aa94440ce31670b90e10fae0af443
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/267681
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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