Run bpf_existence_test in mainline-postsubmit.

The TEST_MAPPING file already contains a mainline-postsubmit
stanza, but according to go/test-mapping-mainline-user-guide
this information also needs to be in AndroidTest.xml (or in
Android.bp, for tests that are not covered by an AndroidTest.xml
and where soong autogenerates the test config).

Test: m bpf_existence_test
Change-Id: I103f5fefbfefa6fd2b59ab6c209fbf1dceada4ba
diff --git a/tests/mts/Android.bp b/tests/mts/Android.bp
index a56f76e..2c44010 100644
--- a/tests/mts/Android.bp
+++ b/tests/mts/Android.bp
@@ -23,6 +23,9 @@
         "general-tests",
         "mts-tethering",
     ],
+    defaults: [
+        "connectivity-mainline-presubmit-cc-defaults",
+    ],
     require_root: true,
     static_libs: [
         "libbase",