Add test for filesystem expansion after snapshot restore / clone

[jsafrane]

As described in container-storage-interface/spec#452, CSI driver should resize filesystem on a volume when restoring a snapshot / cloning a volume to a bigger volume.

csi-test should test this behavior. The test should be optional, at least initially, because we expect a lot of drivers to fail this check.

[jsafrane]

Idea:

1. Create a volume.
2. NodePublish it.
3. Note filesystem size via NodeGetVolumeStats.
4. Take snapshot.
5. Restore the snapshot to a bigger volume.
6. NodePublish it.
7. Compare the current filesystem size via NodeGetVolumeStats.

The comparison must be tolerant to rounding errors or some growth of the filesystem metadata.

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@fejta-bot: Closing this issue.

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