I think that the string is not trimmed in random tests:
Expected: equal to muIcKiTXBKIdGdTwFMFnAxdtEUrRClWvsMPdJxay lPduzTkLMLxay
Actual: muIcKiTXBKIdGdTwFMFnAxdtEUrRClWvsMPdJxay lPduzTkLMLxay

After adding char in front and in end
Expected: equal to PIPay
Actual: PIPay

You're right - my mistake that I didn't do it

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Sure :)

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I'm just wondering why my solution worked in C# (where the timeout was set to 12 seconds if I remember correctly), and the same approach in Java (with a timeout of 16 seconds) did not pass...

main.cpp:101:22: error: 'RomanNumerals' does not refer to a value
Assert::That(RomanNumerals.to_roman(4), Equals(_rnhagm.to_roman(4)));
^

I think that if you wanna us to use instance of RomanNumerals it is not valid to name obj same as whole class... (as I know)
If you wanna us to use static method I believe this should be RomanNumerals::to_roman(...)
Oherwise please provide some basic test in sample tests in the same manner as in attempt to see what you want...

Hmmm... interesting - in C# my solution pass in 12 second timeout :)
But same approach in Java timeouted on 16 second timeout

Any idea why?