Change in flashrom[master]: tests: Add NON_ZERO macro and not_null function instead of MOCK_HANDLE

[Anastasia Klimchuk (Code Review)]

Attention is currently required from: Nico Huber, Angel Pons.
Anastasia Klimchuk has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/57269 )

Change subject: tests: Add NON_ZERO macro and not_null function instead of MOCK_HANDLE
......................................................................

Patch Set 5:

(2 comments)

Patchset:

PS4:
> I wonder if `io_mock` is the right place for it. not_null() doesn't […]

Yes, I was also thinking about whether io_mock is the right header ... so probably it's not.
include/test.h gets LOG_ME further in this chain anyway CB:57914, so I can move non zero and not null there as well.
But I would need include/tests.c for not_null() implementation, is that correct?

Another thing I wanted to ask before I start changing everything: I am using io_mock.c later in this chain for current_io CB:57915 is that correct approach? If yes, I think I can remove io_mock.c from this patch and delay its creation until CB:57915.

Patchset:

PS5: 
Rebased to replace one more occurrence of MOCK_HANDLE from the latest merged test.