Add tests for plugins

[tamalsaha]

reopen #313

[codecov-io]

Codecov Report

Merging #316 into master will increase coverage by 1.41%.
The diff coverage is 22.94%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   30.62%   32.04%   +1.41%     
==========================================
  Files           9       14       +5     
  Lines         542      980     +438     
==========================================
+ Hits          166      314     +148     
- Misses        353      613     +260     
- Partials       23       53      +30

Impacted Files	Coverage Δ
plugins/check_event/lib.go	0% <0%> (ø)
plugins/notifier/sms.go	0% <0%> (ø)	⬆️
plugins/notifier/incident.go	0% <0%> (ø)	⬆️
plugins/notifier/lib.go	1.43% <1.85%> (+1.43%)	⬆️
plugins/check_component_status/lib.go	44.11% <19.44%> (ø)
plugins/check_json_path/lib.go	37.6% <40.9%> (+8.63%)	⬆️
plugins/check_node_status/lib.go	55.55% <42.85%> (-0.9%)	⬇️
plugins/check_cert/lib.go	61.32% <42.85%> (+0.26%)	⬆️
plugins/check_pod_exists/lib.go	41.66% <44.64%> (ø)
plugins/check_node_exists/lib.go	46.03% <47.82%> (ø)
... and 7 more

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