fix(linux): Fix iterator in test backspace handling

Replace reverse iterator loop that holds a stale iterator across
pop_back() calls with a pattern that directly accesses context.back()
on each iteration. This avoids undefined behavior from iterator
invalidation when mutating the vector.

The previous code was similar to the code used in production
(ldml_process.cpp, ldml_event_state::emit_backspace()). However,
in production we use a list where this works, but here in the tests
we use a vector where the behavior is undefined.

Test-bot: skip

Author: ermshiperete

core/tests/unit/ldml/ldml.cpp

@@ -157,21 +157,20 @@ apply_action(
       matched_text = true; // no text to match as context is empty.
     }
     // now, we need to simulate what ldml_processor::emit_backspace() is going to do.
-      auto end = context.rbegin();
-      while (end != context.rend()) {
-        if (end->type == KM_CORE_CT_CHAR) {
-          test_assert(!matched_text);
-          test_assert_equal(end->character, ch); // expect popped char to be same as what's in context
-          matched_text = true;
-          context.pop_back();
-          break;  // exit on first real char
-        }
-        test_assert(end->type != KM_CORE_CT_END);  // inappropriate here.
+    while (!context.empty()) {
+      if (context.back().type == KM_CORE_CT_CHAR) {
+        test_assert(!matched_text);
+        test_assert_equal(context.back().character, ch); // expect popped char to be same as what's in context
+        matched_text = true;
         context.pop_back();
+        break;  // exit on first real char
       }
-      test_assert(matched_text);
+      test_assert(context.back().type != KM_CORE_CT_END);  // inappropriate here.
+      context.pop_back();
     }
+    test_assert(matched_text);
     break;
+  }
   case KM_CORE_IT_PERSIST_OPT:
     std::cout << "  + TODO-LDML: persist_opt()" << std::endl;
     break;