Build/Test Tools: Add end-to-end (e2e) tests README.md.
README.md
Adds a readme to the /tests/e2e/ folder to provide (a) instructions of how to run the tests locally and (b) links to documentation.
/tests/e2e/
Follow-up to [45570].
Props isabel_brison, desrosj, justinahinon. Fixes #53550.