Add --always-clean flag to run-test.

This flag makes run-test remove the test-artifacts even if it fails. Also
enable this option by default when doing run-tests with make. Add
a ART_TEST_RUN_TEST_ALWAYS_CLEAN environment variable to control this
option.

Change-Id: I7867b400d570d8d679d9822d1fa65f49eb3522ae
diff --git a/test/Android.run-test.mk b/test/Android.run-test.mk
index 0f8032e..5c1bc03 100644
--- a/test/Android.run-test.mk
+++ b/test/Android.run-test.mk
@@ -358,6 +358,9 @@
   prereq_rule :=
   skip_test := false
   uc_reloc_type :=
+  ifeq ($(ART_TEST_RUN_TEST_ALWAYS_CLEAN),true)
+    run_test_options += --always-clean
+  endif
   ifeq ($(2),host)
     uc_host_or_target := HOST
     run_test_options += --host