REGTESTS: protobuf: add a test for the ungrpc converter

REGTESTS: protobuf: add a test for the ungrpc converter

ungrpc had no coverage while the protobuf converter is exercised by
nested_and_flat.vtc. It differs from protobuf by the 5-byte gRPC header
it skips first, and by the rule that only the first converter of a
chain may be an ungrpc, the following ones having to be protobuf.

The body is the "Rectangle" message of the documentation example, built
by hand with sendhex: a string field, and two nested messages holding
two varint leaves each. The test extracts the length-delimited field
without a type, the four leaves through their dotted paths, an
intermediary field as a binary sample, and checks that the three
documented equivalent spellings mixing ungrpc and protobuf all return
the same value. A field which is not in the message must yield nothing.