[RFC] make sandbox UT more generic

[AKASHI Takahiro]

Hi,

I'm working on implementing SCMI-based pinctrl/gpio driver,
and want to re-use sandbox UT to test the code. However,
It is somehow sandbox-specific (with additional DT nodes).
How can/should we make it more generic for other targets/drivers
rather than just by copying the test code?
(I have already created a test for pinmux since there is only
one existing scenario, but gpio test has many.)

Even if I say 'generic', my case may be special since real
hardware (device drivers) cannot always run all the test cases,
while SCMI-based drivers potentially can with a dummy SCMI server
for sandbox.
See:
    drivers/firmware/scmi/sandbox-scmi_agent.c

Thanks,
-Takahiro Akashi