Add ART_TEST_RUN_TEST_IMAGE.

To prepare disabling this run on the no-image bot config.

Change-Id: Iebd51761ebde42d07beb7ecce3b84e738d03afe3
diff --git a/build/Android.common_test.mk b/build/Android.common_test.mk
index f335cc3..b7a2379 100644
--- a/build/Android.common_test.mk
+++ b/build/Android.common_test.mk
@@ -96,6 +96,9 @@
 # Do you want run-tests with no prebuilding enabled run?
 ART_TEST_RUN_TEST_NO_PREBUILD ?= $(ART_TEST_FULL)
 
+# Do you want run-tests with a pregenerated core.art?
+ART_TEST_RUN_TEST_IMAGE ?= true
+
 # Do you want run-tests without a pregenerated core.art?
 ART_TEST_RUN_TEST_NO_IMAGE ?= $(ART_TEST_FULL)