Split model_fields tests into different files.

tests/model_fields/test_custom_fields.py

@@ -0,0 +1,22 @@
+from django.db import connection, models
+from django.test import SimpleTestCase
+
+
+class CustomFieldTests(SimpleTestCase):
+
+    def test_get_prep_value_count(self):
+        """
+        Field values are not prepared twice in get_db_prep_lookup() (#14786).
+        """
+        class NoopField(models.TextField):
+            def __init__(self, *args, **kwargs):
+                self.prep_value_count = 0
+                super(NoopField, self).__init__(*args, **kwargs)
+
+            def get_prep_value(self, value):
+                self.prep_value_count += 1
+                return super(NoopField, self).get_prep_value(value)
+
+        field = NoopField()
+        field.get_db_prep_lookup('exact', 'TEST', connection=connection, prepared=False)
+        self.assertEqual(field.prep_value_count, 1)