Add testcase generator

Very simplistics - select M fields, filters on N fields, slight bias to
use values of same type of the field it's comparing against.

No segfaults yet, but one test case that generates differing output when
run against `nulls` and `nulls1`:

```
select rowid from nulls1 where binary_9 >= '56' and ts_5 < 496886400000;
```

tests/test-all

@@ -8,3 +8,4 @@ set -x
 "$here"/test-unsupported
 "$here"/test-supported
 "$here"/test-queries
+"$here"/test-random

tests/test-random

@@ -0,0 +1,100 @@
+#!/usr/bin/python3
+
+import sqlite3
+import os.path
+import random
+
+ops = ['=', '<>', '>', '>=', '<', '<=', 'IS', 'IS NOT', 'LIKE'];
+
+def get_column_values(conn, table, column):
+    cursor = conn.execute('SELECT DISTINCT quote({}) FROM {}'.format(column, table))
+    rv = []
+    for row in cursor:
+        rv.append(row[0])
+    cursor.close()
+    return rv
+
+def get_columns(conn, table):
+    cursor = conn.execute('SELECT * FROM {} LIMIT 1'.format(table))
+    rv = ['rowid'] + [description[0] for description in cursor.description]
+    cursor.close()
+    return rv
+
+def generate_statement(conn, table, column_values, all_values):
+    names = list(column_values.keys())
+
+    num_fields = random.randint(1, 20)
+
+    query = 'SELECT '
+    for i in range(num_fields):
+        if i > 0:
+            query += ', '
+
+        query += 'quote(' + names[random.randint(0, len(names) - 1)] + ')'
+    query += ' FROM '
+    query += table
+
+    num_clauses = random.randint(0, 2)
+    for i in range(num_clauses):
+        if i == 0:
+            query += ' WHERE '
+        else:
+            query += ' AND '
+
+        field = names[random.randint(0, len(names) - 1)]
+        op = ops[random.randint(0, len(ops) - 1)]
+        values = column_values[field]
+        if random.randint(0, 1) == 0:
+            values = all_values
+        value = values[random.randint(0, len(values) - 1)]
+        query += field + ' ' + op + ' ' + str(value)
+
+    return query
+
+def test_statement(conn, table, column_values, all_values):
+    query = generate_statement(conn, table, column_values, all_values)
+
+    gold = [row for row in conn.execute(query)]
+    print('{} rows: {}'.format(len(gold), query))
+    for parquet in ['nulls1', 'nulls2', 'nulls3']:
+        new_query = query.replace('nulls', parquet)
+        rv = [row for row in conn.execute(new_query)]
+        if gold != rv:
+            with open('testcase-cmds.txt', 'w') as f:
+                f.write('.load parquet/libparquet\n.testcase query\n.bail on\n{};\n.output\n'.format(new_query))
+            with open('testcase-expected.txt', 'w') as f:
+                for row in gold:
+                    f.write('{}\n'.format(row))
+            with open('testcase-out.txt', 'w') as f:
+                for row in rv:
+                    f.write('{}\n'.format(row))
+
+            raise ValueError('ruhroh')
+
+
+
+def test_table(conn, table):
+    column_names = get_columns(conn, table)
+    print('Table {}: {}'.format(table, column_names))
+    column_values = {}
+    for name in column_names:
+        column_values[name] = get_column_values(conn, table, name)
+
+    random.seed(0)
+    all_values = []
+    for values in column_values.values():
+        all_values = all_values + values
+    for i in range(1000):
+        test_statement(conn, table, column_values, all_values)
+
+def test_db(db_file, tables):
+    conn = sqlite3.connect(db_file)
+    conn.enable_load_extension(True)
+    conn.load_extension('parquet/libparquet.so')
+    conn.enable_load_extension(False)
+    for table in tables:
+        test_table(conn, table)
+
+if __name__ == '__main__':
+    db_file = os.path.abspath(os.path.join(__file__, '..', '..', 'test.db'))
+    test_db(db_file, ['nulls', 'no_nulls'])