use of `--coverage` with versions >= 3.2

[GitMensch]

@OlegKunitsyn @marcsosduma

GnuCOBOL 3.2, which is to be released today (Oleg, you may want to provide an extra docker container tag for that), added --coverage. The old "manual" approach will still work, but you may want to use the new "simple" one if 3.2+ is detected.