tests: Use direct values instead of raw SQL

Supersedes disqus#11.

In order to prevent (-1) from being masked by BitField.get_prep_value
and converted to 15 (0xf), the test code uses a direct SQL statement.
Its implementation has a few peculiarities that may be undesirable:

- It uses an Django internal API, the Field.column attribute.  Granted,
  this package already uses a lot of internal APIs, and Field.column
  is highly unlikely to change.  However, in general using less internal
  APIs is better for future compatibility.

- Using low-level API misses a lot of code paths that could have been
  tested.

- Neither db_table nor db_column is escaped.  In case we later
  incorporate tests involving pathological SQL object identifiers, we
  have to further use quote_name, which is not exactly public API
  either.

Instead, we use models.Value() with an explicit output_field, which
still avoids BitField.get_prep_value and inserts the value directly.

Further, directly assign to __dict__ so that the BitFieldCreator
descriptor's __set__ method is bypassed and the value is assigned
unchanged.

bitfield/tests/tests.py

@@ -155,11 +155,12 @@ def test_regression_1425(self):
         self.assertTrue(instance.flags.FLAG_2)
         self.assertTrue(instance.flags.FLAG_3)
 
-        cursor = connection.cursor()
-        flags_field = BitFieldTestModel._meta.get_field('flags')
-        flags_db_column = flags_field.db_column or flags_field.name
-        cursor.execute("INSERT INTO %s (%s) VALUES (-1)" % (BitFieldTestModel._meta.db_table, flags_db_column))
-        # There should only be the one row we inserted through the cursor.
+        # Bypass BitField.to_python and insert (-1) directly.
+        instance = BitFieldTestModel()
+        instance.__dict__['flags'] = models.Value(-1, output_field=models.IntegerField())
+        instance.save()
+
+        # There should only be the one row we inserted with a direct value.
         instance = BitFieldTestModel.objects.get(flags=-1)
         self.assertTrue(instance.flags.FLAG_0)
         self.assertTrue(instance.flags.FLAG_1)