Add test_mode to SimulationParameters

[JordiBolibar]

We already have a test flag for some workflows to have specific configurations during tests. However, this should become more systematic, especially following recent issues during tests.

For example, there seems to be an issue when removing workers in GH CI. If there is first a test which uses 4 workers and then comes another one which asks only for 1 worker, the code is made in such a way that 3 workers will be removed. This works fine in local, but seems to crash in GH CI. By using that flag in the parameters, we could avoid removing workers when it test. Such idea could be re-used for many future issues like this.