message: core: adapt locked_queue test
  We can no longer do pops after the queue is empty and we always need to
  borrow before doing a pop.
diff: @@ -21,13 +21,12 @@ TEST(LockedQueue, FillAndEmpty)
  locked_queue.push_back(three);
  EXPECT_EQ(locked_queue.size(), 3);
  + auto tmp = locked_queue.borrow_front();
  locked_queue.pop_front();
  EXPECT_EQ(locked_queue.size(), 2);
  + tmp = locked_queue.borrow_front();
  locked_queue.pop_front();
  - locked_queue.pop_front();
  - EXPECT_EQ(locked_queue.size(), 0);
  -
  - // Popping an empty queue should just fail silently.
  + tmp = locked_queue.borrow_front();
  locked_queue.pop_front();
  EXPECT_EQ(locked_queue.size(), 0);
  }
  @@ -46,14 +45,10 @@ TEST(LockedQueue, BorrowAndReturn)
  auto borrowed_item = locked_queue.borrow_front();
  EXPECT_EQ(*borrowed_item, 1);
  - locked_queue.return_front();
  locked_queue.pop_front();
  borrowed_item = locked_queue.borrow_front();
  EXPECT_EQ(*borrowed_item, 2);
  - locked_queue.return_front();
  - // Double returning shouldn't matter.
  - locked_queue.return_front();
  locked_queue.pop_front();
  borrowed_item = locked_queue.borrow_front();
  