Cleanup / consolidate unit testing code (#4831)

- Move testing code out of helpers.py
- Create new file unit_test.py

InvenTree/users/test_api.py

@@ -3,7 +3,7 @@
 from django.contrib.auth.models import Group, User
 from django.urls import reverse
 
-from InvenTree.api_tester import InvenTreeAPITestCase
+from InvenTree.unit_test import InvenTreeAPITestCase
 
 
 class UserAPITests(InvenTreeAPITestCase):

InvenTree/users/test_migrations.py

@@ -2,14 +2,14 @@
 
 from django_test_migrations.contrib.unittest_case import MigratorTestCase
 
-from InvenTree import helpers
+from InvenTree import unit_test
 
 
 class TestForwardMigrations(MigratorTestCase):
     """Test entire schema migration sequence for the users app."""
 
-    migrate_from = ('users', helpers.getOldestMigrationFile('users'))
-    migrate_to = ('users', helpers.getNewestMigrationFile('users'))
+    migrate_from = ('users', unit_test.getOldestMigrationFile('users'))
+    migrate_to = ('users', unit_test.getNewestMigrationFile('users'))
 
     def prepare(self):
         """Setup the initial state of the database before migrations"""

InvenTree/users/tests.py

@@ -7,7 +7,7 @@ from django.urls import reverse
 
 from rest_framework.authtoken.models import Token
 
-from InvenTree.helpers import InvenTreeTestCase
+from InvenTree.unit_test import InvenTreeTestCase
 from users.models import Owner, RuleSet