Add new unittest to validate current finalizer safety file i/o

[greg7mdp]

• finalizer_tests/finalizer_safety_file_versioning validates loading older versions. Also validate current version: we should be able to load the reference file, save the fsi info, and end up with exactly the same bytes.
• also check that the current version of safety.dat file committed to the repo can be loaded on nodeos startup and saved on nodeos shutdown (assuming no voting or block processing) without changing the contents.

[greg7mdp]

Regarding the second check:

• also check that the current version of safety.dat file committed to the repo can be loaded on nodeos startup and saved on nodeos shutdown (assuming no voting or block processing) without changing the contents.

We can check that the reference file can be loaded by nodeos on startup, but it is saved only when voting (not at nodeos shutdown), so we cannot check "and saved on nodeos shutdown (assuming no voting or block processing) without changing the contents."