Add NetworkStack tests to postsubmit

The NetworkStack tests were unit tests in FrameworksNetTests, but were
moved out so were no longer run in presubmit.
After a while the tests should be moved to presubmit.

Test: atest NetworkStackTests
Change-Id: I06ddcafd6de6d9029ac710f0fc3c4f2d1d705c01
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..55ba591
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "postsubmit": [
+    {
+      "name": "NetworkStackTests"
+    }
+  ]
+}
\ No newline at end of file