Make run-flextesa.sh support Athen and Babylon
With changes in tezos!27 (closed), we can support both Athens and Babylon in the test suite. The interface to use this is:
$ ./ledger/tests/run-flextesa.sh ledger-wallet Athens
“master” branch is used when on Athens, and “zeronet” is used on Babylon. Both use @smondet’s branch for building tezos-sandbox and actually running the tests. A pin to that branch is added as “flextesa-dev”.