These tests can be written by using the test-utils.
Check out the test-utils README that outlines how to write a test.
Take a look at the SharedStringEndToEndTest for a basic example of how to write an end-to-end test.
That same directory contains more complex examples too.
The tests under the real-service-tests dir target a live production service like r11s or ODSP's Fluid server.
These are run via npm run test:realsvc:mocha, and are included in the CI such that a test failure doesn't
fail the pipeline - since a service outage or network hiccup could cause a failure when no code defect is present.