test that MAP_SYNC works

[pohly]

We currently lack tests that verify that the volumes provided by PMEM-CSI really are backed by PMEM. What we promise is that mmap(MAP_SYNC) works, so we should also test exactly that.

We need to test this also with raw block volumes. If possible, just mmap the special device file.

[pohly]

We can implement this as a sanity or E2E test. I'm leaning towards doing it as a proper E2E test. Because we have to run in a cluster, sanity tests aren't that much simpler than a proper E2E test.