WIP: Flextesa: fix tests on MacOSX

Got this to work:

dune build @src/bin_flextesa/runtest

Among the requirements I could reverse-understand:

brew install util-linux
brew install coreutils
export PATH="/usr/local/opt/coreutils/libexec/gnubin:/usr/local/opt/util-linux/bin:/usr/local/opt/util-linux/sbin:$PATH"
ulimit -n 1024

This is one commit on top of tezos/tezos!1044 (closed)