For the time being, we simple try to factorise out the test of
consecutive gao that holds on the same clock.

ps : also fix a pb (infinite loop in SortActions.topo_sort)
introduced in the previous change that, for a strange reason
(Makefile issue), was not catched by the test (almost all test were
failing !!).

pps : it actually seems to break quite a lot of test, but its my test
script that wrong ! since version 527 (we are 534), the compiler used
to perform the test in the tmp dirs is not upadated ! (well it is,
but on the wrong machine...).

I commit this change still as it introduces no more test failures.

Indeed, computing the dependencies and choosing an order for actions
(that may optimize test openning in the near futurea) are 2 different
things.