This commit adds a basic backup/restore test for vmsingle and vmcluster. A
more sophisticated was originally added to the partition index PR
(#8134) and was aimed to test backup/restore when switching back and
forth between legacy and partition index. During the code review it was
decided that it would be good to have a separate test as well since
legacy code will be removed in future and so will the test.
Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>