Add persist.rollback.is_test (6/n)

This property is set to true in rollback tests to prevent
fallback-to-copy when enabling rollbacks by hard linking.

This gives us insights into how hard linking fails where
it shouldn't.

Bug: 168562373
Test: m
Change-Id: Iab22954e9b9da21f0c3c26487cda60b8a1293b47
4 files changed