Add sdkextensions tests to soong TEST_MAPPING

Changes in soong have left sdkextensions tests broken for over 2 days,
so let's prevent that happening again.

Bug: 180105615
Test: presubmit
Change-Id: I6961228bdd5a4606eb5078ed332b508ef6d2d6af
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..9f386ca
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "imports": [
+    {
+      "path": "packages/modules/SdkExtensions"
+    }
+  ]
+}