Add TEST_MAPPING for system/core/rootdir/etc

bionic-unit-tests runs when one of the linker config files is changed.

Test: N/A
Change-Id: I8af1a9728dab4492fe34a3b4daab9c5f9973d84f
diff --git a/rootdir/etc/TEST_MAPPING b/rootdir/etc/TEST_MAPPING
new file mode 100644
index 0000000..af2ec0f
--- /dev/null
+++ b/rootdir/etc/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "bionic-unit-tests"
+    }
+  ]
+}