Method thread

test/exec/asio/test_completion_token.cpp:472–486

    std::thread                                                          t_;
   public:
    thread()
      : g_(ctx_.get_executor())
      , t_(
          [&]() noexcept
          {
            STDEXEC_TRY
            {
              ctx_.run();
            }
            STDEXEC_CATCH_ALL
            {
              FAIL("Exception thrown in background thread");
            }
          })
    {}

    ~thread() noexcept
    {