cmd/internal/testdir: support -godebug

cmd/internal/testdir: support -godebug

Similar with what we are doing for -goexperiment.

For #65778

Change-Id: I7dda69512a3ffb491e3de31941ae1c3d34fececf
Reviewed-on: https://go-review.googlesource.com/c/go/+/568156
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>