This directory contains the infrastructure for running tests on the engine, which are most often run by Flutter's continuous integration (CI) systems.
The tests themselves are located in other directories, closer to the source for each platform, language, and variant. For instance, macOS engine unit tests written in objective C are located in the same directory as the source files, but with a Test suffix added (e.g. “FlutterEngineTest.mm” holds the tests for “FlutterEngine.mm”, and they are located in the same directory).
If you are working on the engine, you will want to be able to run tests locally.
In order to learn the details of how do that, please consult the Flutter Wiki page on the subject.