TIL I totally misunderstood what ExUnit’s
async: true does.
I thought it made ExUnit run the tests in this module/file concurrently.
In fact, it always run tests in this module synchronously, but
:async schedules them in parallel with tests in other files.
Imagine that… reading the docs teaches you things. It’s right there at the top. 😅
:async- configures tests in this module to run concurrently with tests in other modules. Tests in the same module never run concurrently. It should be enabled only if tests do not change any global state. Defaults to false.