Resolve mutation in list_qualifies_for_extra_seat

[Hiddentale]

What & why

Kills a surviving mutation on list_qualifies_for_extra_seat The mutant changes == to != on line 202 of backend/apportionment/src/seat_assignment/residual_seat_assignment.rs making it impossible for a seat to be reassigned to a list that had a seat assigned to it and which then was retracted.
Now if anyone accidentally makes this change in the future, this unit test will flag it by failing.

closes #3446
closes #3355

Reviewer notes

list_qualifies_for_extra_seat(0, Some(1), &[previous_steps], 1u32),
where
number_of_seats_largest_remainders = 0,
number_of_seats_unique_highest_averages = Some(1),
and previous_steps makes sure has_retracted_seat = true,

is the unique combination that makes

(has_retracted_seat && number_of_seats_unique_highest_averages == 1 && number_of_seats_largest_remainders <= 1)

at line 202 of residual_seat_assignment.rs return true.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.17%. Comparing base (0d93e20) to head (0eaf3ff).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3453   +/-   ##
=======================================
  Coverage   90.16%   90.17%           
=======================================
  Files         484      484           
  Lines       23914    23927   +13     
  Branches     2487     2487           
=======================================
+ Hits        21563    21576   +13     
  Misses       2216     2216           
  Partials      135      135           

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