Add tests for completion

[tiangolo]

✅ Add tests for completion.

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 0.65%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   90.96%   91.61%   +0.65%     
==========================================
  Files          47       48       +1     
  Lines        1162     1181      +19     
==========================================
+ Hits         1057     1082      +25     
+ Misses        105       99       -6

Impacted Files	Coverage Δ
typer/completion.py	100% <ø> (+33.33%)	⬆️
tests/test_completion.py	95.45% <95.45%> (ø)

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