Add libgui_test to libs/gui/TEST_MAPPING postsubmit

This should ideally be in presubmit, but from initial investigation it
seems different subsets of this test suite are consistently failing
across different devices. Putting this in (non-blocking) postsubmit
should give us a better view of what's failing where. We can then
fix/ignore tests as needed before moving this to presubmit.

Docs for non-blocking vs. "blocking" (investigated) postsubmit:
go/test-mapping#attributes

Test: will evaluate non-blocking postsubmit failures after this lands
Bug: 257123981
Change-Id: I95a7906eed0b901eb46ab0af6cca47ff5c0d7c45
diff --git a/libs/gui/TEST_MAPPING b/libs/gui/TEST_MAPPING
index 1c43530..9415035 100644
--- a/libs/gui/TEST_MAPPING
+++ b/libs/gui/TEST_MAPPING
@@ -3,5 +3,11 @@
     {
       "path": "frameworks/native/libs/nativewindow"
     }
+  ],
+  "postsubmit": [
+    {
+      // TODO(257123981): move this to presubmit after dealing with existing breakages.
+      "name": "libgui_test"
+    }
   ]
 }