test: Use DEADLINE_TZINFO to validate reminder due date calculation

[jennifer-richards]

Addresses part of #4976

[codecov]

Codecov Report

Merging #5019 (2edc7ed) into main (570bd14) will decrease coverage by 0.01%.
The diff coverage is 98.14%.

@@            Coverage Diff             @@
##             main    #5019      +/-   ##
==========================================
- Coverage   88.47%   88.47%   -0.01%     
==========================================
  Files         296      296              
  Lines       39818    39851      +33     
==========================================
+ Hits        35230    35258      +28     
- Misses       4588     4593       +5     

Impacted Files	Coverage Δ
ietf/meeting/views.py	90.69% <98.00%> (+0.12%)	⬆️
ietf/utils/text.py	87.19% <100.00%> (+1.21%)	⬆️
ietf/meeting/templatetags/proceedings_filters.py	88.23% <0.00%> (-5.89%)	⬇️
ietf/meeting/models.py	85.91% <0.00%> (-0.59%)	⬇️
ietf/group/models.py	92.30% <0.00%> (-0.37%)	⬇️
ietf/doc/views_search.py	89.02% <0.00%> (-0.21%)	⬇️
ietf/group/utils.py	94.17% <0.00%> (+0.44%)	⬆️

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