Selecting the target in pytest

[sjg20]

The pytest code for selecting a target currently looks like this (fixtures.py):

@pytest.fixture(scope="session")
def target(env):
    """Return the default target `main` configured in the supplied
    configuration file."""
    target = env.get_target()
    if target is None:
        raise UserError("Using target fixture without 'main' target in config")

    return target

This works well with a single place, called 'main'. But when there are multiple places, we need a way to select the correct one.

I propose adding a --lg-target option to fixtures.py so that this can be specified.

Then my script to check a particular commit (use with 'git bisect run') can be something like this:

pytest --lg-env env_rpi_try.cfg
--lg-target ${board}
--lg-log --lg-colored-steps
-k test_uboot_smoke

Does this seems reasonable?