go/ast/internal/tests: add missing copyright header
I have forgotten to add it in CL 616340
Change-Id: Ia4753aa20b5e2d9978c129f2ab7d063b63a5b8e2
GitHub-Last-Rev:
2a301b12df2573b1f85eece69175c109b462ab95
GitHub-Pull-Request: golang/go#69766
Reviewed-on: https://go-review.googlesource.com/c/go/+/617616
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>