Stopped special-casing postgres-specific tests

Refs #23879.

tests/postgres_tests/fields.py

@@ -0,0 +1,31 @@
+"""
+Indirection layer for PostgreSQL-specific fields, so the tests don't fail when
+run with a backend other than PostgreSQL.
+"""
+from django.db import models
+
+try:
+    from django.contrib.postgres.fields import (
+        ArrayField, BigIntegerRangeField, DateRangeField, DateTimeRangeField,
+        FloatRangeField, HStoreField, IntegerRangeField,
+    )
+except ImportError:
+    class DummyArrayField(models.Field):
+        def __init__(self, base_field, size=None, **kwargs):
+            super(DummyArrayField, self).__init__(**kwargs)
+
+        def deconstruct(self):
+            name, path, args, kwargs = super(DummyArrayField, self).deconstruct()
+            kwargs.update({
+                'base_field': '',
+                'size': 1,
+            })
+            return name, path, args, kwargs
+
+    ArrayField = DummyArrayField
+    BigIntegerRangeField = models.Field
+    DateRangeField = models.Field
+    DateTimeRangeField = models.Field
+    FloatRangeField = models.Field
+    HStoreField = models.Field
+    IntegerRangeField = models.Field