Enable `NoThunk` in diffusion tests

We either enable `NoThunk` for all the test, or just for one specific simulation. The latter seems to be enough as we will run the same simulation to test different properties.

Start by locating the diffusion tests and the NoThunk setting, then inspect how the simulations are shared across tests. Determine whether enabling NoThunk for one simulation covers the different property checks, and run the relevant diffusion test suite; done means the selected tests pass with NoThunk enabled.

Written by the indexing model from the issue text.