Call fs::u8path to convert some UTF-8 string literals to paths, instead
of relying on implicit conversions. The implicit conversions incorrectly
decode const char* paths using the current windows codepage, instead of
treating them as UTF-8. This could cause test failures depending what
environment windows tests are run in.
Issue was reported by MarcoFalke <falke.marco@gmail.com> in
https://github.com/bitcoin/bitcoin/pull/24306#discussion_r818566106