Update libglnx

• Merge libglnx commit '8f2e676d'

• test-utils: Remove backported g_assert_cmpint, g_assert_cmpstr

  These were in GLib 2.16, which is considerably older than the oldest version we support.

• tests: Don't backport g_assert_cmpmem, g_assert_no_errno

  libglnx now provides better versions of these.

• tests: Use libglnx's newline-safe g_test_message() wrapper

• glib-backports: Drop some things that libglnx now has as backports

• tests: Use libglnx's g_test_skip_printf() backport

• Merge libglnx commit 'e701578c'

• tests: Rely on libglnx's backports of g_assert_true, _false