Convert bitfield tests to use pointers for atomic values

[kannibalox]

This fixes compilation of the tests, which otherwise fails with:

torrent/utils/test_signal_bitfield.cc:128:43: error: could not convert ‘std::bind(_Func&&, _BoundArgs&& ...) [with _Func = bool (*)(atomic<unsigned int>&, unsigned int); _BoundArgs = {unsigned int&, int}; typename _Bind_helper<__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type = _Bind_helper<false, bool (*)(atomic<unsigned int>&, unsigned int), unsigned int&, int>::type; __is_socketlike<_Func> = __is_socketlike<bool (*)(atomic<unsigned int>&, unsigned int), bool (*)(atomic<unsigned int>&, unsigned int)>; typename decay<_Tp>::type = bool (*)(atomic<unsigned int>&, unsigned int)](marked_bitfield, (i % 20))’ from ‘std::_Bind_helper<false, bool (*)(std::atomic<unsigned int>&, unsigned int), unsigned int&, int>::type’ to ‘std::function<bool()>’
  128 |     CPPUNIT_ASSERT(wait_for_true(std::bind(&check_index, marked_bitfield, i % 20)));
      |                                  ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                           |
      |                                           std::_Bind_helper<false, bool (*)(std::atomic<unsigned int>&, unsigned int), unsigned int&, int>::type

[rakshasa]

Not needed anymore.