Allow to specify an arbitrary file in a non-default scenario

[cbeauchesne]

Description

We have actually two ways to execute a test:

./run.sh SCENARIO_NAME   # SCENARIO_NAME is played, and all relevant tests are executed. `SCENARIO_NAME` 
                         # can be ommitted, the default scenario will be executed
./run.sh path/to/a/file  # DEFAULT scenario is played, and every tests inside `path/to/a/file` are executed,
                         # if they belong to the default scenario.

💀 There is no way to execute an arbitrary test if it does not belong the the default scenario.

With this change, it's now possible, by running this :

./run.sh SCENARIO_NAME path/to/a/file

Wait, but why ...

This is not possible:

./run.sh path/to/a/file

And you play the scenario that belong to path/to/a/file ?

For two reasons :

1. the bad one : as now, how system tests works, scenario mechanism must be configured before any test collection. So we can't do it without a pretty big revamp
2. the good one : but what happen if there is more than one scenario in path/to/a/file? running more than one scenario in a pytest process would require a huge revamp.

So for now, let's move forward with this small cost.