I believe there is an error in the example portion of your intructions that lends to some confusion.

Current:
"Putting these results together fusc(10) = fusc(5) = fusc(2) + fusc(3) = 1 + 2 = 3"

Should be:
"Putting these results together fusc(10) = fusc(5) = fusc(2) + fusc(3) = 1 + fusc(1) + fusc(2) = 1 + 1 + 1 = 3"

Even more accurately:
"Putting these results together fusc(10) = fusc(5) = fusc(2) + fusc(3) = fusc(1) + fusc(1) + fusc(2) = 1 + 1 + fusc(1) = 1 + 1 + 1 = 3"

The example should show all the steps of recursion. Currently it appears that fusc(3) directly equals 2 when actually there's another step of recursion that further expands fusc(3) into 2 more functions. I think it's important to show this extra step for clarity.

Ruby translation

Maybe I'm missing a nuance in what is expected, but the C++ test_tree_with_23_nodes test (and later) seem broken. Ex for test_tree_with_23_nodes test:
Expected: [ 12, 13, 16, 1, 40, 29, 2, 19, 6, 35, 3, 38, 6, 22, 29, 8, 10, 10, 8, 21, 1, 36, 33 ]
Actual: [ 22, 8, 35, 3, 13, 29, 38, 1, 6, 10, 19, 29, 33, 36, 40, 1, 2, 6, 8, 10, 12, 16, 21 ]

As you can see, the expected starts out incorrectly sorted after the first level

                                     12
                               *13*      16
                           ...

Lua translation!

Scala translation

The override on runTests is largely required if you want nice output along with only running the last test when all others are green.

Python initial solution:

from typing import List

def tree(root: str, files: List[str]):

typing.List (along with typing.Set, typing.Tuple ...) are deprecated since Python 3.9. Most built-in types like list now directly support type hints: files: list[str]

python new test framework is required. updated in this fork

Scala translation

Anyone have any advice on how to pass the "testRandomTrees" test case ?
hitting error: arrays first differed at element [2]; expected:<8382> but was:<308679>

Not sure how to go about it. Or if anyone had similar issues.

Rust translation

TypeScript translation (based on existing Java Translation)