tests: replace `strcpy()` with `curlx_strcopy()`

tests: replace `strcpy()` with `curlx_strcopy()`

Also:
- examples/hsts-preload: apply the same change as it's based on lib1915
  in tests. Make a local clone of `curlx_strcopy()`. Then drop the
  `_CRT_SECURE_NO_WARNINGS` hack, that's no longer necessary.
- curl_setup.h: delete `strcpy()` from the `_CRT_SECURE_NO_WARNINGS`
  list.

Closes #20076