Fix script parser tests

[phw]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

This is an attempt to fix the tests for metabrainz#2461

When importing from picard.extension_points.script_functions import ext_point_script_functions in one file and then patching picard.extension_points.script_functions import ext_point_script_functions in the tests we ended up with two distinct instances of ExtensionPoint, one in the file that imported ext_point_script_functions and a new one in picard.extension_points.script_functions .

By instead importing import picard.extension_points.script_functions` patching the object inside script_functions makes the patched object available in the importing file as well.