internal/godoc: don't remove test file nodes
internal/godoc: don't remove test file nodes

It's possible that a test file in the main package (as opposed to a
_test package) can have a playable example. So don't remove any
AST nodes from test files.

With this change, FetchModule produces the same doc HTML with and
without the RemoveUnusedAST and InsertPackageSource experiments for
all modules with the top 5000 packages.

Change-Id: Ib003ef5d2fd1f28694953fd414178a1bb8eff965
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260678
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