tests/server: give global `path` variable a more descriptive name

tests/server: give global `path` variable a more descriptive name

Use a more descriptive global variable name in server code, also
to avoid colliding with this name used elsewhere in libcurl.

This isn't causing an issue at this time, but makes the code prone
to `-Wshadow` warnings in unity mode, if the global variable is
compiled first. This specific variable could collide with the `path`
argument of the `curlx_win32_stat()` function.

Closes #16719