Set the Ltac backtrace option in tests.

This is a change enabling backward compatibility.

tests/heap_lang.v

@@ -2,6 +2,7 @@ From iris.program_logic Require Export weakestpre total_weakestpre.
 From iris.heap_lang Require Import lang adequacy proofmode notation.
 (* Import lang *again*. This used to break notation. *)
 From iris.heap_lang Require Import lang.
+Set Ltac Backtrace.
 Set Default Proof Using "Type".
 
 Section tests.

tests/proofmode.v

 From iris.proofmode Require Import tactics intro_patterns.
+Set Ltac Backtrace.
 Set Default Proof Using "Type".
 
 Section tests.

tests/proofmode_iris.v

 From iris.proofmode Require Import tactics monpred.
 From iris.base_logic Require Import base_logic.
 From iris.base_logic.lib Require Import invariants cancelable_invariants na_invariants.
+Set Ltac Backtrace.
 
 Section base_logic_tests.
   Context {M : ucmraT}.

tests/proofmode_monpred.v

 From iris.proofmode Require Import tactics monpred.
+Set Ltac Backtrace.
 
 Section tests.
   Context {I : biIndex} {PROP : sbi}.