test/py: Provide custom IDs when parametrizing tests

When pytest generates the name for parametrized tests, simple parameter
values (ints, strings) get used directly, but more complex values such
as dicts are not handled. This yields test names such as:

    dfu[env__usb_dev_port0-env__dfu_config0]
    dfu[env__usb_dev_port0-env__dfu_config1]

Add some code to extract a custom fixture ID from the fixture values, so
that we end up with meaningful names such as:

    dfu[micro_b-emmc]
    dfu[devport2-ram]

If the boardenv file doesn't define custom names, the code falls back to
the old algorithm.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>