Failure to build with Boost 1.77

Build testing the OpenBSD ports tree with Boost 1.77 beta1. bitcoin 0.21.1 fails to build as shown..

```
wallet/test/init_test_fixture.cpp:34:52: error: no member named 'BOOST_FILESYSTEM_C_STR' in 'boost::filesystem::path'
    std::ofstream f(m_walletdir_path_cases["file"].BOOST_FILESYSTEM_C_STR);
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
1 error generated.
```