test: fix and refactoring force_recovery test

Test checks possibility of recovery with force_recovery option.
For these purpose, snapshot is damaged in test and possibility of
recovery is checked. In previous version snapshot size was too small,
so sometimes in test, system data, needed for recovery was corrupted.
Also moved same code to functions, deleted one of two identical test
case (in previous version we write different count of garbage in the
middle of snapshot, it's meaningless, because result is almost the same,
only the amount of data that can be read from snapshot differs).
Follow-up #5422