Modified is_late to check is_billable

[tbaxter-18f]

Description

Modifies is_late to check for is_billable

Additional information

Also includes some test tidying.

[codecov-io]

Codecov Report

Merging #828 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #828      +/-   ##
=========================================
+ Coverage   90.89%   90.9%   +0.01%     
=========================================
  Files          39      39              
  Lines        1747    1749       +2     
=========================================
+ Hits         1588    1590       +2     
  Misses        159     159

Impacted Files	Coverage Δ
employees/models.py	98.59% <0%> (+0.04%)	⬆️

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[rogeruiz]

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