improve linebreak tests so that they do not rely on infix rendering

4 jobs for gen_proofmode in 27 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #13368
fp
build-coq.8.7.1

00:04:59

passed #13367
fp-timing
build-coq.8.7.2

00:04:27

passed #13366
fp-timing
build-coq.8.8.0

00:04:22

passed #13365
fp
build-coq.dev

00:27:03