Tests fail to compile when `with-http` flag is disabled

[hololeap]

I'm packaging dhall for Gentoo, and it's important for us to include the option for testing if the upstream package includes it. However, our testing (and the build in general) is run in a sandbox with no network access.

Thankfully, there seems to have been an effort to allow for offline testing with the WITH_HTTP cpp symbol functioning as a switch to allow this. 👍

However, it does not seem as though this functionality is currently being tested, and on current master the tests fail to even compile when the with-http flag is disabled.

[Gabriella439]

Fix is up here: #2382