Spinoff from #1702080: Introduce canonical FileStorage + (regular) cache
The reason this is necessary is because #1702080-58: Introduce canonical FileStorage + (regular) cache passed despite it contained a fatal error in encode / decode ergo there is no testing for the current storage decode/encode.
These methods are tested as part of ConfigStorageTestBase.
However, dedicated test coverage might be good idea.
I think that the tests in ConfigStorageTestBase especially now that #1653026: [META] Use properly typed values in module configuration has landed extensively test encode and decode
Drupal is a registered trademark of Dries Buytaert.