Nothing really doable about that: when the output is wrong, expected and actual are printed to the console, which is limited. Considering the size of the matrices, any wrong code will saturate the buffer. Nothing to do about that. You won't see the problem if you have a working code.

The only problem is if you use a language which is showing the expected result even when the test is passing... => is that the case? (not closing to keep the edit panel accessible)

Your code is too slow.

Why is the 'abcd' only response expected here

abcd is a list with all the expected permutations, not a literal string "abcd".

All the other problems come from your solution being wrong. Not an issue.