use the correct windows path separator in test file

spiffworkflow-backend/tests/data/xml_with_entity/invoice.bpmn

@@ -1,5 +1,5 @@
 <!--?xml version="1.0" ?-->
-<!DOCTYPE replace [<!ENTITY ent SYSTEM "file://{{FULL_PATH_TO_FILE}}"> ]>
+<!DOCTYPE replace [<!ENTITY ent SYSTEM "{{FULL_PATH_TO_FILE}}"> ]>
 <userInfo>
  <firstName>John</firstName>
  <lastName>&ent;</lastName>

spiffworkflow-backend/tests/spiffworkflow_backend/unit/test_spec_file_service.py

@@ -249,6 +249,9 @@ class TestSpecFileService(BaseTest):
         tmp_file = os.path.normpath(
             self.get_test_data_file_full_path("file_to_inject", "xml_with_entity")
         )
+
+        # add the file: with the correct separator for windows
+        tmp_file = f"file:{os.path.sep}{tmp_file}"
         file_contents = self.get_test_data_file_contents(
             "invoice.bpmn", "xml_with_entity"
         )