Add unit test for formula calculation engine

[schbook]

Support the "inlineStr" cell type

[codecov-io]

Codecov Report

Merging #209 into master will decrease coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   98.49%   98.43%   -0.06%     
==========================================
  Files          16       16              
  Lines        3450     3452       +2     
==========================================
  Hits         3398     3398              
- Misses         32       34       +2     
  Partials       20       20

Impacted Files	Coverage Δ
rows.go	98.91% <0%> (-1.09%)	⬇️

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (7216935) to head (01d2b3a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   99.46%   99.54%   +0.08%     
==========================================
  Files          32       32              
  Lines       25761    25756       -5     
==========================================
+ Hits        25622    25640      +18     
+ Misses         75       60      -15     
+ Partials       64       56       -8     

Flag	Coverage Δ
unittests	99.54% <100.00%> (+0.08%)	⬆️

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[xuri]

LGTM, I add unit test for formula calculation engine based on this branch.