gotest: explicitly skip type descriptors

gotest: explicitly skip type descriptors

Type descriptors are normally weak and nm will report them as V,
so we will skip them when collecting the list of symbols.
But when not using GNU nm, they may be reported as D,
so also skip them in symstogo.

This fixes go/doc/check on Solaris.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/215900