Add Test case and fix bug

[xinyee20]

Update lesson when student info changes
Add testcase for flagatt command

[codecov-io]

Codecov Report

Merging #280 into master will increase coverage by 2.01%.
The diff coverage is 66.05%.

@@             Coverage Diff              @@
##             master     #280      +/-   ##
============================================
+ Coverage     49.75%   51.76%   +2.01%     
- Complexity      804      855      +51     
============================================
  Files           155      155              
  Lines          3871     3931      +60     
  Branches        460      471      +11     
============================================
+ Hits           1926     2035     +109     
+ Misses         1805     1750      -55     
- Partials        140      146       +6     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/team/serenity/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...a/team/serenity/logic/commands/ViewAttCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../team/serenity/logic/commands/ViewFlagCommand.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...a/team/serenity/logic/commands/ViewGrpCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...team/serenity/logic/commands/ViewScoreCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...erenity/logic/commands/question/ViewQnCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...nity/logic/commands/student/DelStudentCommand.java	85.10% <0.00%> (ø)	13.00 <0.00> (ø)
...ty/logic/commands/studentinfo/AddScoreCommand.java	23.94% <0.00%> (-0.70%)	4.00 <0.00> (ø)
...logic/commands/studentinfo/MarkPresentCommand.java	93.24% <ø> (-0.18%)	22.00 <0.00> (ø)
...ty/logic/commands/studentinfo/SetScoreCommand.java	25.75% <0.00%> (-0.81%)	4.00 <0.00> (ø)
... and 33 more

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