VTS: Add vts-core to the dependency of tests

Bug: 137921421
Test: (1) m tests
      (2) check vts-core-tests.zip

For migrating most existing VTS test to executable by Base TradeFed, we have to
create vts-core suite and test all of the modules in vts-core first.
And add the packaging rules for vts-core suite.

Bug: 137921421
Change-Id: Ifc92db205c78db98abf704aa6dfa50f1f77e983d
diff --git a/core/tasks/vts-core-tests.mk b/core/tasks/vts-core-tests.mk
index 919354c..fb1e1c6 100644
--- a/core/tasks/vts-core-tests.mk
+++ b/core/tasks/vts-core-tests.mk
@@ -45,3 +45,5 @@
 
 vts-core: $(vts-core-zip)
 $(call dist-for-goals, vts-core, $(vts-core-zip) $(vts-core-list-zip) $(vts-core-configs-zip))
+
+tests: vts-core