Improve Test Coverage and add test files

[YuunoKun]

Improve Test Coverage.
Add new EnterSessionCommand/EnterSessionCommandParser test files

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 4.60%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #100      +/-   ##
============================================
+ Coverage     71.35%   75.96%   +4.60%     
- Complexity      655      718      +63     
============================================
  Files           109      109              
  Lines          2067     2068       +1     
  Branches        225      225              
============================================
+ Hits           1475     1571      +96     
+ Misses          524      442      -82     
+ Partials         68       55      -13     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/atas/commons/core/Messages.java	75.00% <ø> (ø)	2.00 <0.00> (ø)
...logic/commands/sessionlist/EditSessionCommand.java	92.85% <ø> (+7.14%)	14.00 <0.00> (ø)
...gic/commands/sessionlist/ClearSessionsCommand.java	100.00% <100.00%> (ø)	3.00 <1.00> (+1.00)
...gic/commands/sessionlist/DeleteSessionCommand.java	100.00% <100.00%> (+38.46%)	8.00 <1.00> (+5.00)
...mmands/sessionlist/session/ParticipateCommand.java	94.11% <100.00%> (+11.76%)	8.00 <0.00> (+1.00)
.../commands/sessionlist/session/PresenceCommand.java	94.44% <100.00%> (+11.11%)	8.00 <0.00> (+1.00)
...ttendance/exceptions/SessionNotFoundException.java	100.00% <100.00%> (+100.00%)	1.00 <1.00> (+1.00)
src/main/java/atas/logic/parser/ParserUtil.java	97.67% <0.00%> (+4.65%)	17.00% <0.00%> (+1.00%)
...rc/main/java/atas/model/attendance/Attributes.java	85.71% <0.00%> (+4.76%)	11.00% <0.00%> (+1.00%)
... and 16 more

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[nweiyue]

LGTM! Nice coverage for the test cases

[erisjacey]

LGTM