Document --keep-snapshots in our test doc
Parent task: #15864
I am learning how to run the test suite. In various parts of the documentation there are referencies to "snapshots", but the option
--keep-snapshots is never suggested in https://tails.boum.org/contribute/release_process/test/usage/
I understand that it is common to use it when developing, so it might be suggested; I also guess that it could need lots of disk space, so maybe that should be documented, too.