tests: Run the proto2 half of the protobuf conformance suite

The suite only built conformance.proto and test_messages_proto3.proto,
so the testee answered every proto2 request with "Unknown message
protobuf_test_messages.proto2.TestAllTypesProto2". Skipped tests do not
fail a conformance run, so 1301 tests counted as passing by being
absent.

Vendoring test_messages_proto2.proto takes the suite from 1366 successes
with 1301 skips to 2643 successes with none, and exposes six real
failures: an int32 JSON parse gap shared with proto3, a oneof duplicate
that should be rejected, and three MessageSet cases that need group
support.

Those six only exist for a runner that templates the JSON suite over
both syntaxes, which 26.1 was the first to do, so they go into the 33.4
failure list alone. The 21.9 list needs no entries: that runner drives
proto2 through the binary suite and one recommended JSON test, all of
which pass.

Note that this does not cover proto2 required fields. The suite never
drives TestAllRequiredTypesProto2, so their behavior stays invisible
here; the differential test against libprotobuf covers it instead.

TEXT_FORMAT remains unimplemented, so 434 text-format tests still skip.

Task-number: QTBUG-150055
Pick-to: 6.12
Change-Id: I35c2b53b4eff62d64d12ddf00b258a3a94d629ed
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>