Add test_mainline_modules option support in mk

Bug: 165425972
Test: Apply aosp/1418212
      atest -b hello_world_test
      Check module-info.json for hello_world_test HelloWorldTests and
HelloWorldHostTest

Change-Id: Ie30a09a54b0ae927ca4d8d1aaba2ac0cdb15233a
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 9c29974..d515db3 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -300,6 +300,7 @@
 LOCAL_TARGET_REQUIRED_MODULES:=
 LOCAL_TEST_CONFIG:=
 LOCAL_TEST_DATA:=
+LOCAL_TEST_MAINLINE_MODULES:=
 LOCAL_TEST_MODULE_TO_PROGUARD_WITH:=
 LOCAL_TIDY:=
 LOCAL_TIDY_CHECKS:=