PARQUET-113: Add more array backward-compatibility tests.
PARQUET-113: Add backward-compat for parquet-avro list.

This adds backward-compatibility for the case where parquet-avro wrote
an ambiguous structure annotated by LIST. The Avro schema in the file's
metadata shows that the one-field record contained in the list should be
interpreted as an actual record, rather than ignored as a synthetic
group for the 3-level representation. This can also be triggered by
setting the Avro read schema.

Now the only case where parquet-avro will not correctly read data
written by parquet-avro is when the synthetic list is present and the
Avro schema also contains a single-field record and that field is named
"element". This case is extremely unlikely.