Assemble and pretrain require an output argument. This commit adds
assemble testing, but not pretrain, as that requires an actual trainable
component, which is not currently in the test config.
The code argument imports the provided file. If it adds item to the
registry, that affects global state, which CliRunner doesn't isolate.
Since there's no standard way to remove things from the registry, this
instead uses subprocess.run to run commands.
One tricky thing here: it's tempting to create the config by creating a
pipeline in code, but that requires declaring the custom components
here. However the CliRunner appears to be run in the same process or
otherwise have access to our registry, so it works even without any
code arguments. So it's necessary to avoid declaring the components in
the tests.
* fix processing of "auto" in walk_directory
* add check for None
* move AUTO check to convert and fix verification of args
* add specific CLI test with CliRunner
* cleanup
* more cleanup
* update docstring
