CI: harden test pty error handling

[apostasie]

This is a debugging PR to help figure out and fix issues outlined in #3952

There is likely two distinct issues:

• the first one is likely a race between copying from the master and what the container writes out - it is possibly because of the hack necessary because of icmd use of MultiWriter
• the second one is more mysterious: it seems like in certain circumstances the container exits immediately (?)

I will likely restart the CI many times to try and reproduce the issues.

[apostasie]

Ok, it is consistently green.

Good to merge.