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