internal/frontend: add page-content tests for non-redistributable module
internal/frontend: add page-content tests for non-redistributable module

Also, strengthen the "readme" test: it was succeeding just because
the page says "tab=readme" somewhere.

Updates b/139286026.

Change-Id: I99975549aa2fe664da7c710c9de9a05bb83c539f
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/527174
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

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