Add MicrodroidHostTestCases to TEST_MAPPING

Microdroid shares init with its host, defined under system/core/init.
This change adds MicrodroidHostTestCases to TEST_MAPPING to ensure that
Microdroid tests can verify patches to init.

Test: presubmit
Change-Id: I55f7e3ffbaf3401d145afac5f7c53d83f69e6ba7

init/TEST_MAPPING

@@ -5,6 +5,9 @@
     },
     {
       "name": "init_kill_services_test"
+    },
+    {
+      "name": "MicrodroidHostTestCases"
     }
   ]
 }