Runs a requirement's tests as surfex reads them (Surfex.Select, §11): every test whose
verifies: declarations resolve to the unit, a list's members included.
mix surfex.test cart-add # the tests declaring it
mix surfex.test spec.md#carts --within # and those of the units inside it
mix surfex.test cart-add --list # name them; run nothing
mix surfex.test cart-add -- --trace # what follows `--` goes to mix testmix test --only verifies:cart-add selects a test tagged ["cart-add", "carts"] by
neither id: ExUnit compares a list tag as one string. This task selects by declaration
and runs mix test FILE:LINE … in a child process, which ExUnit's location filter
selects exactly, then exits with that run's status. The run records its evidence as any
run does. Each unit is any reference a tag may use. A unit that is unknown, ambiguous,
or that no test declares is an error before anything runs.