Add schema import tests

Interesting, this query previously returned an error:

```
SELECT * FROM clickhouse_bin.types2 ORDER BY c1 LIMIT 2;
ERROR:  clickhouse_fdw: LowCardinality(Nullable) is not supported
```

But not it returns the values:

```
SELECT * FROM clickhouse_bin.types2 ORDER BY c1 LIMIT 2;
     c1
-------------
 cardinal 1
 cardinal 10
(2 rows)
```

Which, in my testing, is exactly what ClickHouse itself returns, given
the same columns and rows. I presume this reflects an improvement in the
updated clickhouse-cpp library (the test uses the binary client for the
`clickhouse_bin` schema).

Author: theory