Add more unit tests

[LinuxSuRen]

No description provided.

[codecov]

Codecov Report

Merging #279 (9e8a68a) into master (6475b7a) will increase coverage by 8.80%.
The diff coverage is 65.90%.

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   25.86%   34.66%   +8.80%     
==========================================
  Files          28       30       +2     
  Lines        1918     2002      +84     
==========================================
+ Hits          496      694     +198     
+ Misses       1397     1277     -120     
- Partials       25       31       +6     

Flag	Coverage Δ
unittests	34.66% <65.90%> (+8.80%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/exec/command.go	0.00% <0.00%> (ø)
pkg/http.go	0.00% <ø> (ø)
pkg/installer/fetch.go	12.62% <ø> (ø)
pkg/installer/process.go	0.00% <0.00%> (ø)
cmd/get.go	68.30% <25.00%> (+35.45%)	⬆️
pkg/installer/check.go	26.53% <25.00%> (+2.62%)	⬆️
pkg/net/http.go	44.70% <50.00%> (+12.92%)	⬆️
cmd/root.go	76.47% <57.14%> (-6.29%)	⬇️
cmd/install.go	34.18% <100.00%> (+13.58%)	⬆️
pkg/compress/bzip2.go	72.00% <100.00%> (+72.00%)	⬆️
... and 16 more

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