Add tests for Rust protover::compute_for_old_tor() and C functions it calls

In https://trac.torproject.org/projects/tor/ticket/25386#comment:21, after changing/adding some tests, I noticed that protover::compute_for_old_tor() always returns an empty string, meaning that the version we parsed is new enough that the router should be reporting its own protocol versions. It's because the Rust code is calling the C version of tor_version_as_new_as(), which expects a platform string, not a version, so it gets to tor_version_parse_platform(), decides this is a "non-standard tor" and returns true early. So, in the top of the Rust function protover::compute_for_old_tor(), when we call tor_version_as_new_as(version, FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS) it says true, and we never reach the rest of the function.

The solution is to prepend "Tor " to each of the query strings in protover::compute_for_old_tor().