Add an option to rm the snapshots with golden_run

[cedelavergne-ledger]

When using the option golden_run, the snapshots are re-generated, but if we've modified the tests, and the number of png is lower than before, the old useless png are not deleted.
For example, taking a test that produces 10 screens, from 00000.png to 00009.png; after a modification, there are now only 8 files: 00008.png and 00009.png should then be deleted.

It could be usefull to have an option allowing to erase the snapshots directory before running the test, to ensure no more useless files are present.

[xchapron-ledger]

At some point this wasn't possible because we were chaining multiple navigate_and_compare call with the same dir but with hardcoded start_idx.
I'm not sure it's still the case?