Support non-gtest NATIVE_TESTS

We've had this in Soong for a while, but never supported it in Make.

Bug: 33819328
Test: Change LOCAL_GTEST on a module, check m -j ... showcommands
Change-Id: Id8f4b9908c82c05d9a986a38945b33bac578e991
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c0343a0..6e61d15 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -78,6 +78,7 @@
 LOCAL_GENERATED_SOURCES:=
 # Group static libraries with "-Wl,--start-group" and "-Wl,--end-group" when linking.
 LOCAL_GROUP_STATIC_LIBRARIES:=
+LOCAL_GTEST:=true
 LOCAL_HAL_STATIC_LIBRARIES:=
 LOCAL_INIT_RC:=
 LOCAL_INSTALLED_MODULE:=