[M] Change in flashrom[master]: flashrom_tester: Change String types to PathBuf where appropriate

[Edward O'Callaghan (Code Review)]

Attention is currently required from: Evan Benn.

Edward O'Callaghan has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/69064 )

Change subject: flashrom_tester: Change String types to PathBuf where appropriate
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Patch Set 1:

(2 comments)

Commit Message:

https://review.coreboot.org/c/flashrom/+/69064/comment/b2bc64b9_83d70e7c 
PS1, Line 7: String
`str` not `String`, a `String` is a heap allocated "string" while a `str` is a stack allocated "string". Typically the type was `str` with a stack life-time variable `a`.

https://review.coreboot.org/c/flashrom/+/69064/comment/61c8fc40_7cf59cb9 
PS1, Line 9: String 
s/String/Stack allocated str's with life-time 'a'/