Question: what is the benefit of this extra layer of abstraction?

We set default values for all parameters, and could skip them in some test cases.

Are there cases where a parameter can be omitted and default values used instead?
Even so, it is more transparent to have an explicit indication that the input is “empty” data than to do so implicitly.

Are there cases where a parameter can be omitted and default values used instead?

Yes. I removed this code from places, where it does not needed.

Do we really need to hide a set of arguments in the variable? It doesn't seem transparent to understand.

Here I found the case, when two sets of the parameters are used. I just decided not to delete this. Maybe it makes the code overloaded, I agree. Do you think I should remove it?

1. As for me, it makes the code a bit difficult to understand.
2. It complicates the ability to make local changes, as there is an unnecessary binding to a variable that can be used elsewhere, and you need to maintain consistency across different places.

Removed.

Do I understand correctly that cat_result is the return code of the external utility?
It looks like we are testing the operation of this utility. It should be enough to check whether it succeeds or not, and with what code it doesn't matter.
Therefore, it would be more correct to split the test - one for successful cases, the other for error cases.

return code of the external utility?

Nope, this is result of tt cat <args> command execution.

split the test

Now they divided to common part and timestamp testing part. I thought that a good idea not to mix them.

You're right about the “good idea not to mix them”, that's the reason why it's better to stick with that idea to the end and separate them into successful and unsuccessful ones as well.
So it would be something like:

• common success
• common fail
• timestamp success
• timestamp fail

This separation will make the code simpler, remove unnecessary parameterization at the input and unnecessary flow control (if-else) inside the test function.

I divided tests by arguments and results.

In Python is dangerous the use a mutable data types for the default value.

Agree, my mistake. Fixed.