Bug #43291 MTR2: it is not possible to obtain error log for a full test run

Description:
It used to be possible to use testsuite according to this scenario:

* Amend the code so that on some "interesting" condition it prints relevant information to stderr.
* Run the entire test suite.
* Look into the log files and see if/when the interesting condition was encountered.

With MTR2 it is not possible anymore - it seems the logs are erased, one can only find the logs of the last testcase.

How to repeat:
Put somewhere in the code 

     fprintf(stderr, ".....);

run the testsuite, look at the log files.

Suggested fix:
Make it possible to save .err logs (maybe as an option).

I think this is the same issue as is being addressed in Bug #40978.

There's an option to put DBUG_ASSERT(0) instead of fprintf(stderr, ...);   and then look for test failures but its substantially worse as it requires one to catch one "interesting case" at a time.

I don't understand the last comment. Is the problem that the .err file from the server is overwritten?  That's what the bug I referred to is about.

Set to "Need feedback" because feedback about if this is duplicate of bug #40978 or not is needed.

No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Duplicate of Bug #40970, as previously commented