proofreadpage_tests.py: Fix variable name
proofreadpage_tests.py: Fix variable name

Fix variable name:
- page_text -> loaded_text

Change-Id: I7c8cdc2c79e53eecb54f002f356c6d3a7034b623

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core