add missing tests for Monitor.java

[simonsymhoven]

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[codecov]

Codecov Report

Merging #17 (0a35267) into master (9d87295) will increase coverage by 44.96%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             master      #17       +/-   ##
=============================================
+ Coverage     48.64%   93.61%   +44.96%     
  Complexity        5        5               
=============================================
  Files             3        2        -1     
  Lines            74       47       -27     
  Branches          4        3        -1     
=============================================
+ Hits             36       44        +8     
+ Misses           36        2       -34     
+ Partials          2        1        -1     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/jenkins/plugins/monitoring/Monitor.java	95.65% <100.00%> (+23.55%)	5.00 <0.00> (+1.00)
...enkins/plugins/monitoring/MonitorUserProperty.java

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