How to deal with readonly_files

[casparvl]

• We'll encourage the use of readonly_files in tests (e.g. in reviews), because most tests actually don't write to the files in their sourcedir
• We can implement an option to quickly mark all files in the sourcesdir as readonly. E.g. test developer would set readonly_files = 'all', then the EESSI_Mixin class, in it's class body, would list all files in the sourcesdir and overwrite readonly_files with that list.
• We'll initialize readonly_files = None in the EESSI_Mixin class body. Then, check after init if it's been set to anything else. If not, we print an error from EESSI_Mixin telling users that they have to set it explicitely (e.g. to [] if they don't want any readonly files).