I thought I did :) There are two options:
Either wait for the package to appear in updates-testing and update it without specifying any URLs:
sudo dnf update --enablerepo=updates-testing curl
... or install it by explicit Koji URLs but then you need to specify all the dependent subpackages, too:
sudo dnf install https://kojipkgs.fedoraproject.org//packages/curl/7.59.0/6.fc28/x86_64/curl-7.59.0-6.fc28.x86_64.rpm https://kojipkgs.fedoraproject.org//packages/curl/7.59.0/6.fc28/x86_64/libcurl-7.59.0-6.fc28.x86_64.rpm https://kojipkgs.fedoraproject.org//packages/curl/7.59.0/6.fc28/x86_64/libcurl-devel-7.59.0-6.fc28.x86_64.rpm
You got the failure because because libcurl-7.59.0-6.fc28 was just not available in the updates-testing repository you were using. If you specify the packages using Koji URLs explicitly, you need to do that for all sub-packages, too. Otherwise you end up with broken dependencies like this. This is not a bug in the packages themselves.