fix unstable widget test on windows/ubuntu

[sensuikan1973]

resolve #46 partially

[codecov-io]

Codecov Report

Merging #47 (6e9f907) into main (a422a10) will decrease coverage by 4.00%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   76.46%   72.45%   -4.01%     
==========================================
  Files          28       28              
  Lines         667      668       +1     
==========================================
- Hits          510      484      -26     
- Misses        157      184      +27     

Impacted Files	Coverage Δ
lib/engine/edax_asset.dart	79.16% <50.00%> (-4.17%)	⬇️
lib/engine/options/book_file_option.dart	77.77% <50.00%> (ø)
lib/engine/options/eval_file_option.dart	44.44% <50.00%> (ø)
lib/board/pedax_board.dart	85.71% <100.00%> (-3.33%)	⬇️
lib/home/book_file_path_setting_dialog.dart	90.00% <100.00%> (-10.00%)	⬇️
lib/board/square.dart	64.44% <0.00%> (-24.45%)	⬇️
lib/home/level_setting_dialog.dart	86.36% <0.00%> (-13.64%)	⬇️
lib/home/n_tasks_setting_dialog.dart	86.95% <0.00%> (-13.05%)	⬇️
... and 1 more