[Robustness] Add first robustness tests

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Add two tests:
- TestManySmallFiles: writes 100k files size 4k to a directory. Snapshots the data tree, restores and validates data.
- TestModifyWorkload: Loops over a simple randomized workload. Performs a series of random file writes to some random sub-directories, then takes a snapshot of the data tree. All snapshots taken during this test are restore-verified at the end.

A global test engine is instantiated in main_test.go, to be used in the robustness test suite across tests (saves time loading/saving metadata once per run instead of per test).

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