tests: add test for mall HTTP POST

[midgleyc]

Test for #639.

[codecov]

Codecov Report

Merging #651 (599e2e8) into main (f601b52) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #651      +/-   ##
============================================
+ Coverage     21.47%   21.49%   +0.01%     
- Complexity     9243     9247       +4     
============================================
  Files          1003     1003              
  Lines        156729   156729              
  Branches      34794    34794              
============================================
+ Hits          33658    33683      +25     
+ Misses       116847   116816      -31     
- Partials       6224     6230       +6     

Impacted Files	Coverage Δ
...rceforge/kolmafia/persistence/HolidayDatabase.java	34.57% <0.00%> (ø)
...sourceforge/kolmafia/session/EquipmentManager.java	22.49% <0.00%> (+0.08%)	⬆️
...eforge/kolmafia/persistence/MallPriceDatabase.java	45.09% <0.00%> (+15.68%)	⬆️

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