Previously, the rust implementation of protover considered an empty string to be
a valid ProtoEntry, while the C version did not (it must have a "=" character).
Other differences include that unknown protocols must now be parsed as
`protover::UnknownProtocol`s, and hence their entries as
`protover::UnvalidatedProtoEntry`s, whereas before (nearly) all protoentries
could be parsed regardless of how erroneous they might be considered by the C
version.

My apologies for this somewhat messy and difficult to read commit, if any part
is frustrating to the reviewer, please feel free to ask me to split this into
smaller changes (possibly hard to do, since so much changed), or ask me to
comment on a specific line/change and clarify how/when the behaviours differ.

The tests here should more closely match the behaviours exhibited by the C
implementation, but I do not yet personally guarantee they match precisely.

 * REFACTOR unittests in protover::protover.
 * ADD new integration tests for previously untested behaviour.
 * FIXES part of #24031: https://bugs.torproject.org/24031.