cmd/go, testing: add TB.ArtifactDir and -artifacts flag
Add TB.ArtifactDir, which returns a directory for a test to store
output files in. Add a -artifacts testflag which enables persistent
storage of artifacts in the output directory (-outputdir, or the
current directory by default).
Fixes #71287
Change-Id: I5f6515a6cd6c103f88588f4c033d5ea11ffd0c3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/696399
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>