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Add a test file (currently with a single test) for manifest v3 related
sanity checks, where we can exercise core functionality of extensions
and make sure it works with manifest v3. This isn't intended to replace
more targeted and robust tests for specific features, and instead to
just serve as a suite of end-to-end integration tests.

Currently, the single test is of programmatic script injection with
a service worker-based extension.

Bug: 993912
Change-Id: I35406ac73ab3ea50d152509068191a68b5fccb87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754529Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687398}