tests/posix/head: improve head's tests

[mssun]

Add some tests for common functions which are not covered by existing cases.

[codecov]

Codecov Report

Merging #23 into master will increase coverage by 0.8%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #23     +/-   ##
=========================================
+ Coverage   21.67%   22.48%   +0.8%     
=========================================
  Files          25       25             
  Lines        2966     2966             
  Branches      681      681             
=========================================
+ Hits          643      667     +24     
+ Misses       2171     2134     -37     
- Partials      152      165     +13

Impacted Files	Coverage Δ
src/posix/head/mod.rs	65.1% <0%> (+12.49%)	⬆️

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