Update dummy-spreadsheet.xlsx add bg-colour test

[zackteo]

Resolves #19

[codecov-io]

Codecov Report

Merging #22 (64ba295) into develop (3979812) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop      #22      +/-   ##
===========================================
- Coverage    99.18%   98.98%   -0.21%     
===========================================
  Files            9        9              
  Lines          492      492              
  Branches         4        4              
===========================================
- Hits           488      487       -1     
- Misses           0        1       +1     
  Partials         4        4              

Impacted Files	Coverage Δ
src/zero_one/fxl/read_xlsx.clj	97.50% <0.00%> (-1.25%)	⬇️

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