Fixed #18317 -- Removed db specific raw SQL function from tests

A test in model_fields used LEN() in raw SQL. This function is not
available on some 3rd party backends. I removed this function and
ensured that the test works correctly (breaks pre e9bbdb39de) with
the change.

tests/regressiontests/model_fields/tests.py

@@ -208,7 +208,7 @@ class BooleanFieldTests(unittest.TestCase):
         # Verify that when an extra clause exists, the boolean
         # conversions are applied with an offset
         b5 = BooleanModel.objects.all().extra(
-            select={'string_length': 'LENGTH(string)'})[0]
+            select={'string_col': 'string'})[0]
         self.assertFalse(isinstance(b5.pk, bool))
 
 class ChoicesTests(test.TestCase):