Change in flashrom[master]: tests: Mock file i/o for linux_mtd and linux_spi tests

[Angel Pons (Code Review)]

Attention is currently required from: Nico Huber, Edward O'Callaghan, Anastasia Klimchuk.
Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/56413 )

Change subject: tests: Mock file i/o for linux_mtd and linux_spi tests
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Patch Set 6:

(2 comments)

File tests/init_shutdown.c:

https://review.coreboot.org/c/flashrom/+/56413/comment/50c606e9_0a72b1b3 
PS4, Line 214: 	int num_bytes = 0;
> Wow thank you for the whole code! I only removed 
> `const char *data = NULL;`
> it seemed unused.

Well spotted, it was indeed unused. I decided to use an array of structs halfway through and didn't realize that `data` was no longer needed.

File tests/init_shutdown.c:

https://review.coreboot.org/c/flashrom/+/56413/comment/170e2324_8a7ace8c 
PS6, Line 237: 			return strlen(entry.data);
Hmmm, now that I take a look at this again:

- Shouldn't we honor the `size` and `len` parameters?
- `strcpy()` also copies the null terminator, so the returned value should be `strlen(entry.data) + 1`.