Ensure vts_libsnapshot_test will pass after being interrupted.

We try to clean up previous test runs, but this can crash since we
haven't opened fake-super yet. Refactor the harness so we always open
fake-super if it exists. If it does, we'll delete and recreate it after
cleanup. If it doesn't, we'll create it immediately.

It's still possible that cleanup can fail: If interrupted during a merge,
libsnapshot does not allow cleanup until the merge completes. The test
harness doesn't bother handling this case yet.

Bug: 187151854
Test: vts_libsnapshot_test, ctrl+c, run again
Change-Id: I58a7094336a391cff493a31e4f80d8c8b1b166f8