cmd/internal/testdir: accept build go1.x build tag
While at it, also using "slices" package to simplify code.
For #63489
Change-Id: I72b325f6ad379b996c108145885fa71706f6659f
Reviewed-on: https://go-review.googlesource.com/c/go/+/536055 Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>