test&cov: add test case

[xtx1130]

• add test case for 'ENOENT'
• ignore some useless decision logics

[codecov]

Codecov Report

Merging #1211 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1211      +/-   ##
=========================================
+ Coverage   99.73%    100%   +0.26%     
=========================================
  Files           5       5              
  Lines         384     383       -1     
=========================================
  Hits          383     383              
+ Misses          1       0       -1

Impacted Files	Coverage Δ
lib/context.js	100% <ø> (+2.85%)	⬆️

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