Replaces fs-err in snapshot_utils tests

[brooksprumo]

Problem

We're trying to remove the fs-err crate. For more information, please refer to #34838

Summary of Changes

Replace fs_err in snapshot_utils.rs with std::fs

Note, all the uses of fs-err here are in tests, and are always unwrapped or asserted. So nothing that'll be displayed/logged for a user.

[brooksprumo]

This is very similar to #34861, but that PR was for snapshot_bank_utils.rs, and this is for snapshot_utils.rs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f9bfb60) 81.7% compared to head (50477d2) 81.7%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #34870   +/-   ##
=======================================
  Coverage    81.7%    81.7%           
=======================================
  Files         826      826           
  Lines      223371   223371           
=======================================
  Hits       182578   182578           
  Misses      40793    40793