all: ensure all files have licenses, add license test
all: ensure all files have licenses, add license test

Change-Id: I4eb00a7f8bbf7bfaa2037cf448ccfd0effb225eb
Reviewed-on: https://code-review.googlesource.com/5220
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

googleapis/cloud-bigtable-cbt-cli,googleapis/cloud-bigtable-cbt-cli