I have had this imaptest[1] stall. I compared this command with what you have on the website and I was a bit surprised that what is on the website is working. What I have noticed is that if you remove this msgs argument, the imaptest stalls. I am not sure if this is according to design, but one might expect that specifying a secs, makes specifying msgs redundant?

[1] imaptest - append=100,0 logout=0 host=xx.local port=143 user=xxx pass=xxx seed=100 secs=240 clients=1 mbox=64kb.mbox box=inbox/test.

[2] https://doc.dovecot.org/admin_manual/sample_tests/

Using: dovecot-imaptest-2.3.10-2.x86_64