REGTESTS: protobuf: cover the ungrpc message realignment

REGTESTS: protobuf: cover the ungrpc message realignment

Regression test for 3a319e14c ("BUG/MINOR: sample: realign the ungrpc
converter on message boundaries").

grpc.vtc already walks a body of two gRPC messages, but the lookup
succeeds in its first message, so the cursor is left at the message end
and the bug does not show: that file passes both before and after the
fix. Reaching the defect needs a lookup that fails part-way through the
first message, which a length-delimited field whose wire type does not
match the requested one produces.

The body is 23 bytes and holds field 1 twice: length-delimited in the
first message, with a payload that reads as a gRPC header once the
cursor is misaligned, and as a varint 9999 in the second one.
ungrpc(1,int64) must return 9999. Before the fix it returned 1337, a
field of no message of the body.

Checked against 3a319e14c (passes) and its parent 1f114e4c4 (fails,
returning 1337).