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
diff --git a/private/shell.te b/private/shell.te
index 3ca8360..f6ad966 100644
--- a/private/shell.te
+++ b/private/shell.te
@@ -94,6 +94,9 @@
 # userspace reboot
 set_prop(shell, userspace_reboot_test_prop)
 
+# Allow shell to set this property used for rollback tests
+set_prop(shell, rollback_test_prop)
+
 # Allow shell to get encryption policy of /data/local/tmp/, for CTS
 allowxperm shell shell_data_file:dir ioctl {
   FS_IOC_GET_ENCRYPTION_POLICY