Speed-up scripts/verify-tff-mapping by avoiding shell out

What does this MR do and why?

Speed-up scripts/verify-tff-mapping by avoiding shell out.

Verify mapping by using TestFileFinder API instead.

The runtime of scripts/verify-tff-mapping went down from 14s to 0.5s.

Discovered while working on !119316 (merged).

How to set up and validate locally

time scripts/verify-tff-mapping

Before

tff mapping verification passed.

real    0m13.543s
user    0m10.923s
sys     0m2.603s

After

tff mapping verification passed.

real    0m0.530s
user    0m0.467s
sys     0m0.081s

MR acceptance checklist

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