No side effects in function arguments

We want to be able to see tests with eqc_gen:sample, we also want to be
able to re-run tests.

Side effects are no good idea if these goals need to be met. Therefore,
we replace the side effect to ask for a port by making a symbolic call
of this.

Nicer solutions are possible, but at least we can now re-run the test
case, and therefore shrink.
Reduce the total testing time for the properties to 20 seconds

We need to make a global budget for testing on CI server and then
define for each property how much it may use of that time. Probably in
auxiliary file, such that it can be computed from what has changed in
the repo.