Fix reporting terminal dimensions

[MisterDA]

• Fix division by zero when size of the terminal is incorrectly reported as zero. (fix #356, #381, @MisterDA)

• Enable terminal size reporting on macOS and Windows. Also report the terminal size even when the test is run buffered by Dune. (#381, @MisterDA)

• Allow overriding the number of columns with ALCOTEST_COLUMNS env var. (#322, #381, @MisterDA)

My intent was to use a bigger default than 80 columns when the CI is detected. Sadly, this would seem to require too big of a refactoring to communicate the default size to the Pp module, which is also why I haven't made the number of columns configurable programmatically, and exposed it only via an environment variable to fix reproducibility issues for the Unix platform. Oddly enough, communicating the bounds to the Format module with Format.set_margin hides the first [OK], [FAIL], etc tag. I've haven't dug why. I've tested that this on macOS, Windows FreeBSD, and Linux.

[MisterDA]

@OlivierNicole has done a nice review for this PR, thanks. I think it can be merged as-is. Does a anyone have an opinion?