Instead of curl.haxx.se
Also widen the .gitignore for libtest, since it missed libtest751,
so ignore three digit tests that start with 5-9 instead of just 5-6.
Closes#17502
Before this patch, the result code was a mixture of `int` and
`CURLcode`.
Also adjust casts and fix a couple of minor issues found along the way.
Cherry-picked from #13489Closes#13600