leds: led-test: Fill out the registration test to cover more test cases

leds: led-test: Fill out the registration test to cover more test cases

Upon successful LED class device registration, it is expected that
certain attributes are filled out in pre-defined ways.  For instance, if
provided, the .brightness_get() call-back should be called to populate
the class device 'brightness' attribute, 'max_brightness' should be
initialised as LED_FULL (at least until we can rid these pesky enums)
and the sysfs group should be created with the class device name
supplied by the caller.

If subsequent registrations take place that would result in name
conflicts, various outcomes are expected depending on which flags are
set.  If LED_REJECT_NAME_CONFLICT is disabled, registration should
succeed resulting in an iteration on the provided name.  Conversely, if
it's enabled, then registration is expected to fail outright.

We test for all of these scenarios here.

Link: https://lore.kernel.org/r/20250501081918.3621432-2-lee@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>