migrate echo and http-return-json-headers test helper hosts

[hellais]

This issue is about migrating:

• mia-echoth.ooni.nu (37.218.241.93) 7.40 -> keep or migrate, yet migration requires writing code to support this legacy codebase
• mia-httpth.ooni.nu (37.218.241.94) 7.40 -> keep or migrate, yet migration requires writing code to support this legacy codebase

We will probably have to re-implement these using go or python since the legacy version is running on a very old unsupported version of twisted.

[hellais]

If we move this rewriting the legacy code we could get rid of this issue too: ooni/sysadmin#128