tests: Add tests of snapshots

Add three tests which test the snapshot feature:

* SanityChecks.test_snapshot
      A simple test which verifies copy-on-write semantics of snapshots.
* ShortSystemTests.test_snapshot_goal
      This test verifies if chunks are kept with proper number of
      copies, ie. maximum goal of files the chunk belongs to.
* ShortSystemTests.test_mfsmakesnapshot
      This test checks if various invocations of mfsmakesnapshot work
      as expected.

Change-Id: Ic2edf1080342a7fa5f1b06c1336a5932fbb9a8bf