Merge: tests: add --autosav to automatically update the sav directory.

Advantages:

* avoid manual error-prone update (especially on large tests)
* diffs on the sav/ files may be easier to see with git tools

Drawbacks:

* the wrong sav dir can by used for specific engines/uname/etc.
* manual inspection still required

Is used for #1253 because manual updates are a PITA

Pull-Request: #1258
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>