Improve low cardinality indices detection

[macbre]

And fix integration tests

Examples

-- Server version:		8.0.20 MySQL Community Server - GPL

mysql> select TABLE_NAME, INDEX_NAME, COLUMN_NAME, CARDINALITY from INFORMATION_SCHEMA.STATISTICS where table_name = '0020_big_table';
+----------------+------------+-------------+-------------+
| TABLE_NAME     | INDEX_NAME | COLUMN_NAME | CARDINALITY |
+----------------+------------+-------------+-------------+
| 0020_big_table | num_idx    | num         |           2 |
| 0020_big_table | PRIMARY    | item_id     |      100256 |
| 0020_big_table | text_idx   | text        |      100256 |
+----------------+------------+-------------+-------------+
3 rows in set (0.00 sec)

vs

-- Server version:		5.5.5-10.5.4-MariaDB-1:10.5.4+maria~focal mariadb.org binary distribution

mysql> select TABLE_NAME, INDEX_NAME, COLUMN_NAME, CARDINALITY from INFORMATION_SCHEMA.STATISTICS where table_name = '0020_big_table';
+----------------+------------+-------------+-------------+
| TABLE_NAME     | INDEX_NAME | COLUMN_NAME | CARDINALITY |
+----------------+------------+-------------+-------------+
| 0020_big_table | PRIMARY    | item_id     |      100256 |
| 0020_big_table | text_idx   | text        |      100256 |
| 0020_big_table | num_idx    | num         |           4 |
+----------------+------------+-------------+-------------+
3 rows in set (0.00 sec)

indexdigest/test/linters/test_0031_low_cardinality_index.py:17: AssertionError
----------------------------- Captured stdout call -----------------------------
get_low_cardinality_indices [{'TABLE_NAME': '0020_big_table', 'INDEX_NAME': 'PRIMARY', 'COLUMN_NAME': 'item_id', 'CARDINALITY': 100000}, {'TABLE_NAME': '0020_big_table', 'INDEX_NAME': 'text_idx', 'COLUMN_NAME': 'text', 'CARDINALITY': 100000}, {'TABLE_NAME': '0020_big_table', 'INDEX_NAME': 'num_idx', 'COLUMN_NAME': 'num', 'CARDINALITY': 6}]