test1188: Check for --fail HTTP status

test1188: Check for --fail HTTP status

- Change the test to check for curl error on HTTP 404 Not Found.

test1188 tests "--write-out with %{onerror} and %{urlnum} to stderr".
Prior to this change it did that by specifying a non-existent host which
would cause an error. ISPs may hijack DNS and resolve non-existent hosts
so the test would not work if that was the case.

Ref: https://en.wikipedia.org/wiki/DNS_hijacking#Manipulation_by_ISPs
Ref: https://github.com/curl/curl/issues/6621
Ref: https://github.com/curl/curl/pull/6623

Closes https://github.com/curl/curl/pull/6637