Don't fall back to copy during rollback tests (5/n)

1. 'persist.rollback.is_test' is set to 1 during rollback tests
2. don't fall back to copy if hardlink fails during rollback tests
3. This gives us insight into how enable-rollback-by-hardlink works
   and how it fails in the field

Bug: 168562373
Test: atest MultiUserRollbackTest \
            NetworkStagedRollbackTest \
            RollbackTest \
            StagedRollbackTest
Change-Id: If1590fdf96e25105936a930b3ec7225ecec4bf0d