cmd/go: add documentation for test and xtest fields output by go list

The TestEmbedPatterns, TestEmbedFiles, XTestEmbedPatterns, and
XTestEmbedFiles fields were left out of golang.org/cl/282195 which was
supposed to document the embed fields available in the go list
output. Add documentation for them in this CL.

Fixes #43081

Change-Id: Ifc256c476daec7c0f0e2c41f86b82f958b3e2b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/284258


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>