The storage_engine was not actually a configurable parameter (#101)

without getting the parameter from the different configuration options, the value is actually hardcoded and cannot be changed by the user.

pymongo_inmemory/context.py

@@ -127,7 +127,7 @@ def __init__(
         )
         self.archive_folder = mkdir_ifnot_exist(self.download_folder, self.url_hash)
         self.extracted_folder = mkdir_ifnot_exist(self.extract_folder, self.url_hash)
-        self.storage_engine = "ephemeralForTest"
+        self.storage_engine = conf("storage_engine", "ephemeralForTest")
 
     def __str__(self):
         return (