Add presubmit test for vendor overlay

Add a test that creates files in the appropriate vendor_overlay directory and
verifies that they are correctly overlaid (or not) onto /vendor after rebooting.

Test: locally running atest
Change-Id: I65860dbeb837f86ac030fa51b3af93844e82de96
diff --git a/TEST_MAPPING b/TEST_MAPPING
index cc85408..716378b 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -10,6 +10,9 @@
       "name": "fs_mgr_unit_test"
     },
     {
+      "name": "fs_mgr_vendor_overlay_test"
+    },
+    {
       "name": "init_tests"
     },
     {