hpc: Fix encoding issues. Add test for and fix #17073

* Make sure files are being read/written in UTF-8. Set encoding while writing
  HTML output. Also set encoding while writing and reading .tix files although
  we don't yet have a ticket complaining that this poses problems.
* Set encoding in html header to utf8
* Upgrade to new version of 'hpc' library and reuse `readFileUtf8`
  and `writeFileUtf8` functions
* Update git submodule for `hpc`
* Bump up `hpc` executable version

Co-authored-by: Ben Gamari <ben@smart-cactus.org>