By default, cargo encourages developers to put integration test files directly
in the /tests directory. Doing so means that a different test crate (and
associated binary) is built for each file.
There is both a compile-time and a runtime cost to doing this, as (1) multiple
different binaries must be built to run all tests and (2) while cargo can
run multiple tests in parallel, it runs multiple binaries sequentially.
By instead having a single crate which contains all tests (as we do in
/tests/integration), we save on compilation time, as well as being able to
fully parallelise all of our test runs.
See this blog post for more information.