[bugfix] Use a copy of failed_dep_script_set for _expected_failures
[bugfix] Use a copy of failed_dep_script_set for _expected_failures

failed_dep_script_set is a mutable. Assigning it to _expected_failures and
modifying _expected_failures also modifies failed_dep_script_set's content
because _expected_failures is just a reference and using
failed_dep_script_set with the next class it unexpectly uses the modified
content and tests may fail.

Therefore use a copy.

Bug: T209762
Change-Id: I2df96be60b9aaaf462f843a6fd6b2d9e0b33368e

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