Add couple of tests for compat module (#4845)

tests/test_compat.py

+from django.test import TestCase
+
+from rest_framework import compat
+
+
+class CompatTests(TestCase):
+
+    def test_total_seconds(self):
+        class MockTimedelta(object):
+            days = 1
+            seconds = 1
+            microseconds = 100
+        timedelta = MockTimedelta()
+        expected = (timedelta.days * 86400.0) + float(timedelta.seconds) + (timedelta.microseconds / 1000000.0)
+        assert compat.total_seconds(timedelta) == expected
+
+    def test_get_remote_field_with_old_django_version(self):
+        class MockField(object):
+            rel = 'example_rel'
+        original_django_version = compat.django.VERSION
+        compat.django.VERSION = (1, 8)
+        assert compat.get_remote_field(MockField(), default='default_value') == 'example_rel'
+        assert compat.get_remote_field(object(), default='default_value') == 'default_value'
+        compat.django.VERSION = original_django_version
+
+    def test_get_remote_field_with_new_django_version(self):
+        class MockField(object):
+            remote_field = 'example_remote_field'
+        original_django_version = compat.django.VERSION
+        compat.django.VERSION = (1, 10)
+        assert compat.get_remote_field(MockField(), default='default_value') == 'example_remote_field'
+        assert compat.get_remote_field(object(), default='default_value') == 'default_value'
+        compat.django.VERSION = original_django_version
+
+    def test_patch_in_http_method_names(self):
+        assert 'patch' in compat.View.http_method_names
+
+    def test_set_rollback_for_transaction_in_managed_mode(self):
+        class MockTransaction(object):
+            called_rollback = False
+            called_leave_transaction_management = False
+
+            def is_managed(self):
+                return True
+
+            def is_dirty(self):
+                return True
+
+            def rollback(self):
+                self.called_rollback = True
+
+            def leave_transaction_management(self):
+                self.called_leave_transaction_management = True
+
+        original_transaction = compat.transaction
+        dirty_mock_transaction = MockTransaction()
+        compat.transaction = dirty_mock_transaction
+        compat.set_rollback()
+        assert dirty_mock_transaction.called_rollback is True
+        assert dirty_mock_transaction.called_leave_transaction_management is True
+
+        clean_mock_transaction = MockTransaction()
+        clean_mock_transaction.is_dirty = lambda: False
+        compat.transaction = clean_mock_transaction
+        compat.set_rollback()
+        assert clean_mock_transaction.called_rollback is False
+        assert clean_mock_transaction.called_leave_transaction_management is True
+
+        compat.transaction = original_transaction