BOOST_TEST_EQ does not print C strings

`BOOST_TEST_EQ` that involve C-style strings do not output string contents. For example, `BOOST_TEST_EQ(*++itr, "foo");` shows this on failure:

```
libs\filesystem\test\path_test.cpp(341): test '*++itr == "foo"' ('"c:"' == '00007FF79AE87414') failed in function 'void __cdecl `anonymous-namespace'::iterator_tests(void)'
```

This seems to be caused by this `test_output_impl` overload:

https://github.com/boostorg/core/blob/e53393357f32078f55cdb291a536bb567895bc62/include/boost/core/lightweight_test.hpp#L167

Is this intended to return `const void*` instead of `const char*`?

If yes, then could we make this behavior optional?