libsnapshot_test: Fix running on DSUs.

Because DSUs mount userdata via a fiemap, libfiemap has trouble creating
additional fiemaps on top of it. The complex stacking of dm-linear is
not supported. For other libfiemap tests we've hacked around this
limitation. If LpMetadata is in a folder named "test", we allow the
backing device search to stop at a dm node, whereas otherwise it would
need to stop at a physical device.

However this was not quite enough for vts_libsnapshot_test, because (1)
the test folder was not included in the pattern match, and (2)
CreateLogicalPartition() could not handle device-mapper names, as it
expects a named physical partition. Addressing both of these allows the
tests to pass on DSUs.

Bug: 156713441
Test: vts_libsnapshot_test on DSU
Change-Id: Ie7ee70e31dff0809a5f0c402ed132d80dd03d9b1