Adds helper function for testing a file download via the api

InvenTree/InvenTree/api_tester.py

@@ -2,6 +2,10 @@
 Helper functions for performing API unit tests
 """
 
+import io
+import re
+
+from django.http.response import StreamingHttpResponse
 from django.contrib.auth import get_user_model
 from django.contrib.auth.models import Group
 from rest_framework.test import APITestCase
@@ -165,3 +169,33 @@ class InvenTreeAPITestCase(APITestCase):
             self.assertEqual(response.status_code, expected_code)
 
         return response
+
+    def download_file(self, url, data, expected_code=None, expected_fn=None):
+        """
+        Download a file from the server, and return an in-memory file
+        """
+
+        response = self.client.get(url, data=data, format='json')
+
+        if expected_code is not None:
+            self.assertEqual(response.status_code, expected_code)
+
+        # Check that the response is of the correct type
+        if not isinstance(response, StreamingHttpResponse):
+            raise ValueError("Response is not a StreamingHttpResponse object as expected")
+        
+        # Extract filename
+        disposition = response.headers['Content-Disposition']
+
+        result = re.search(r'attachment; filename="([\w.]+)"', disposition)
+
+        fn = result.groups()[0]
+
+        with io.BytesIO() as fo:
+            fo.name = fn
+            fo.write(response.getvalue())
+        
+        if expected_fn is not None:
+            self.assertEqual(expected_fn, fn)
+        
+        return fo