Merge #3100

3100: network-mux: fix a deadlock r=coot a=coot

* fill completion var with an application error (mux used to ignore application errors)
* fix a deadlock, includes a test that can reproduce it (without the proposed fix)
* a bit of refactoring, the testing module deserves more love

Co-authored-by: Marcin Szamotulski <[email protected]>

Author: iohk-bors[bot]