[test][multidomain] Change pressure entries type to double.

What this MR does / why does DuMux need it:

The pressure value should be handled as double instead of int. Having them as int can lead to undesired behaviour.

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)