Add ExitCommand tests

[jordanyoong]

No description provided.

[codecov-io]

Codecov Report

Merging #96 into master will increase coverage by 12.22%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             master      #96       +/-   ##
=============================================
+ Coverage     35.22%   47.45%   +12.22%     
- Complexity      201      276       +75     
=============================================
  Files            75       75               
  Lines          1374     1374               
  Branches        127      127               
=============================================
+ Hits            484      652      +168     
+ Misses          855      646      -209     
- Partials         35       76       +41     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/commands/AddCommand.java	78.57% <0.00%> (ø)	5.00% <0.00%> (+1.00%)
...ava/seedu/address/model/account/entry/Revenue.java	61.11% <0.00%> (+5.55%)	3.00% <0.00%> (+1.00%)
...rc/main/java/seedu/address/model/account/Name.java	70.00% <0.00%> (+10.00%)	4.00% <0.00%> (+2.00%)
src/main/java/seedu/address/model/CommonCents.java	58.62% <0.00%> (+10.34%)	9.00% <0.00%> (+1.00%)
...seedu/address/model/account/UniqueAccountList.java	37.03% <0.00%> (+11.11%)	9.00% <0.00%> (+2.00%)
...rc/main/java/seedu/address/model/ModelManager.java	46.00% <0.00%> (+12.00%)	11.00% <0.00%> (+3.00%)
...seedu/address/model/account/entry/RevenueList.java	26.66% <0.00%> (+13.33%)	4.00% <0.00%> (+3.00%)
...ava/seedu/address/model/account/entry/Expense.java	72.22% <0.00%> (+16.66%)	4.00% <0.00%> (+2.00%)
.../seedu/address/commons/core/category/Category.java	66.66% <0.00%> (+16.66%)	5.00% <0.00%> (+2.00%)
...seedu/address/model/account/entry/ExpenseList.java	36.66% <0.00%> (+23.33%)	5.00% <0.00%> (+4.00%)
... and 15 more

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[Ziyang-98]

LGTM