If set, the contents of this field are run as a Python script before any tests within that directory are executed. This happens once per execution directory, so tests in subdirectories should take care. The execution of the setup script gets a global environment like lit.local.cfg, but with shared_output as an additional global. This is a path that can be shared by all files within a particular directory, located alongside the %t paths within the Output/ directories. It's also available in tests as %shared_output.
This is useful for families of tests that have some expensive setup, which would be bad to duplicate across tests.
Either this or D35396 will need to be rebased on top of the other.