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
diff --git a/tests/RollbackTest/MultiUserRollbackTest.xml b/tests/RollbackTest/MultiUserRollbackTest.xml
index 2f62af1..8fa0510 100644
--- a/tests/RollbackTest/MultiUserRollbackTest.xml
+++ b/tests/RollbackTest/MultiUserRollbackTest.xml
@@ -20,6 +20,8 @@
         <option name="run-command" value="pm uninstall com.android.cts.install.lib.testapp.B" />
         <option name="teardown-command" value="pm uninstall com.android.cts.install.lib.testapp.A" />
         <option name="teardown-command" value="pm uninstall com.android.cts.install.lib.testapp.B" />
+        <option name="run-command" value="setprop persist.rollback.is_test 1" />
+        <option name="teardown-command" value="setprop persist.rollback.is_test 0" />
     </target_preparer>
     <test class="com.android.tradefed.testtype.HostTest" >
         <option name="class" value="com.android.tests.rollback.host.MultiUserRollbackTest" />