adjust atomic test for more Coq compatibility

tests/atomic.ref

@@ -15,8 +15,12 @@
 "non_laterable"
      : string
 The command has indeed failed with message:
+Ltac call to "iAuIntro" failed.
 Tactic failure: iAuIntro: not all spatial assumptions are laterable.
 The command has indeed failed with message:
+In nested Ltac calls to "awp_apply (open_constr)",
+"<ssreflect_plugin::ssrtclseq@0> wp_apply_core lem fun H => iApplyHyp H ; last  iAuIntro" and
+"iAuIntro", last call failed.
 Tactic failure: iAuIntro: not all spatial assumptions are laterable.
 "printing"
      : string

tests/atomic.v

@@ -2,6 +2,7 @@ From iris.proofmode Require Import tactics.
 From iris.program_logic Require Export atomic.
 From iris.heap_lang Require Export lifting notation.
 From iris.heap_lang Require Import proofmode notation atomic_heap.
+Set Ltac Backtrace.
 Set Default Proof Using "Type".
 
 Section tests.