tests: improve documentation

Closes #711
Je propose d'utiliser l'italique ou le blockquote pour les blagues.

* `testall.sh` run some tests on all engines
   
   > no more excuse to forget to check your new tests with an obscure engine

vs

* `testall.sh` run some tests on all engines

   *(no more excuse to forget to check your new tests with an obscure engine)*