Merge: tests: specific mass-definition of tests marked todo

The magic string "NOT YET IMPLEMENTED" that is used to mark tests as *todo* are now extendable as new magic strings can be added to the `todo` file (one per line)
Moreover, specific engines and platforms can define additional magic strings in the `todo` files in their `sav` subdirectories.
E.g. `sav/Darwin/todo` for magic strings that will be used only for tests run on max os x.

Pull-Request: #1626
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>