Added additional tests for TabArray

[leifgehrmann]

• Tests were missing for three lines of code in Implement TabArray class #50:
  • Testing that the the setter/getter methods for Layout work.
  • Checking that IndexError is raised when setting an incorrect number of decimal_point values.
• Updated _get_tab in Layout to return an optional TabArray if none is set.
• Updated _set_tab in Layout to allow passing in None.
• Added an acceptance test to demonstrate how TabArrays work.
• Renamed the files and variable names tabarray to tab_array, just for consistency with how other object files are named. This should not impact how the code is actually used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1d12c01) 98.90% compared to head (feba606) 99.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   98.90%   99.30%   +0.40%     
==========================================
  Files          20       20              
  Lines        1004     1008       +4     
==========================================
+ Hits          993     1001       +8     
+ Misses         11        7       -4     

Files	Coverage Δ
pangocffi/__init__.py	86.04% <100.00%> (ø)
pangocffi/layout.py	100.00% <100.00%> (+2.75%)	⬆️
pangocffi/tab_array.py	100.00% <ø> (ø)

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