Block Editor: Add a script to run Gutenberg end 2 end tests on Core.

The script is not part of CI yet because the job takes 40mn to run
and it's not useful in all PRs/commits.

Fixes #50469.

Built from https://develop.svn.wordpress.org/trunk@48382


git-svn-id: https://core.svn.wordpress.org/trunk@48151 1a063a9b-81f0-0310-95a4-ce76da25c4cd