System tests: Ignore missing reference files and export diff (#583)

New preCICE versions might add new types of exported files, which should not trigger an error. This already happened with the `.vtk.series` files.

Adding also the new `--diff` option to export the differences of the files.

tools/tests/docker-compose.field_compare.template.yaml

@@ -7,7 +7,4 @@ services:
     command:
       - /runs/{{ tutorial_folder }}/{{ precice_output_folder }}
       - /runs/{{ tutorial_folder }}/{{ reference_output_folder }}
-      - "-rtol 3e-7"
-
-# Currently its really hard to estimate the impact of compiling and executing in a different platform (like github actions)
-# 3e-7 might not be the thightest we can afford and we want to have but its an okayish guestimation for now. 
+      - "-rtol 3e-7 --ignore-missing-reference-files --diff"