Cleanup / consolidate unit testing code (#4831)

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

InvenTree/build/test_api.py

@@ -11,7 +11,7 @@ from build.models import Build, BuildItem
 from stock.models import StockItem
 
 from InvenTree.status_codes import BuildStatus, StockStatus
-from InvenTree.api_tester import InvenTreeAPITestCase
+from InvenTree.unit_test import InvenTreeAPITestCase
 
 
 class TestBuildAPI(InvenTreeAPITestCase):

InvenTree/build/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 build app."""
 
-    migrate_from = ('build', helpers.getOldestMigrationFile('build'))
-    migrate_to = ('build', helpers.getNewestMigrationFile('build'))
+    migrate_from = ('build', unit_test.getOldestMigrationFile('build'))
+    migrate_to = ('build', unit_test.getNewestMigrationFile('build'))
 
     def prepare(self):
         """Create initial data!"""
@@ -58,7 +58,7 @@ class TestForwardMigrations(MigratorTestCase):
 class TestReferenceMigration(MigratorTestCase):
     """Test custom migration which adds 'reference' field to Build model."""
 
-    migrate_from = ('build', helpers.getOldestMigrationFile('build'))
+    migrate_from = ('build', unit_test.getOldestMigrationFile('build'))
     migrate_to = ('build', '0018_build_reference')
 
     def prepare(self):
@@ -113,7 +113,7 @@ class TestReferencePatternMigration(MigratorTestCase):
     """
 
     migrate_from = ('build', '0019_auto_20201019_1302')
-    migrate_to = ('build', helpers.getNewestMigrationFile('build'))
+    migrate_to = ('build', unit_test.getNewestMigrationFile('build'))
 
     def prepare(self):
         """Create some initial data prior to migration"""

InvenTree/build/tests.py

@@ -4,7 +4,7 @@ from django.urls import reverse
 
 from datetime import datetime, timedelta
 
-from InvenTree.helpers import InvenTreeTestCase
+from InvenTree.unit_test import InvenTreeTestCase
 
 from .models import Build
 from stock.models import StockItem