LWT should support wide strings in BOOST_TEST_EQ

Outputting wide strings to `std::cout` outputs the `void*` under C++ before 20, and hits a deleted `op<<` in C++20. We should make it work, either by transcoding to UTF-8, or by using the `codecvt` facet of the output stream.

This might entail adding an adapter such as `boost::core::narrow(x)` that is similar to `quoted(x)` in that when output to a stream, it does the necessary narrowing using `codecvt`. (We can't add it to `io` because Core is not allowed to depend on Io.)

Related: we don't have a wide equivalent of `BOOST_TEST_CSTR_EQ`.