tests: Run reasoner in new process for each formula
As it stands, cool leaks tons of memory with each `isSat` call. Until
this is fixed, the new `--slow` testsuite would need at least 16GB of
memory if all formulas are decided in one process. Thus, we now call
isSat in a new child-process and allow the OS to reclaim memory after
the child exits.
NB: This uses the Unix.fork API, which is not available on Windows.