Build System: Add variable for copying test config

If LOCAL_TEST_CONFIG is set, it will be the default test config
and ignore the local AndroidTest.xml.

BUG: 110982517
Test: vi art/tools/ahat/Android.mk
      set LOCAL_TEST_CONFIG := ahat_test.xml
      add ahat_test.xml in local
      m -j ahat;
      Then, out/host/linux-x86/testcases/ahat/ahat.config
      and ahat_test.xml should be the same.

Change-Id: I2f4ef735fca7d523bb637a18075512bdebf9826f
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index e7116a5..1c45b2b 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -266,6 +266,7 @@
 LOCAL_STRIP_MODULE:=
 LOCAL_SYSTEM_SHARED_LIBRARIES:=none
 LOCAL_TARGET_REQUIRED_MODULES:=
+LOCAL_TEST_CONFIG:=
 LOCAL_TEST_CONFIG_OPTIONS:=
 LOCAL_TEST_DATA:=
 LOCAL_TEST_MODULE_TO_PROGUARD_WITH:=