Add more tests

[yyutong]

No description provided.

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by 8.90%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #118      +/-   ##
============================================
+ Coverage     28.82%   37.73%   +8.90%     
- Complexity      184      240      +56     
============================================
  Files            81       81              
  Lines          1332     1333       +1     
  Branches        117      117              
============================================
+ Hits            384      503     +119     
+ Misses          907      771     -136     
- Partials         41       59      +18     

Impacted Files	Coverage Δ	Complexity Δ
.../seedu/address/logic/parser/ExpenseBookParser.java	60.00% <100.00%> (+60.00%)	8.00 <0.00> (+8.00)
.../java/seedu/address/model/expense/Description.java	44.44% <0.00%> (ø)	2.00% <0.00%> (+1.00%)
...main/java/seedu/address/model/expense/Expense.java	32.07% <0.00%> (+3.77%)	10.00% <0.00%> (+1.00%)
.../java/seedu/address/model/ExpenseModelManager.java	30.76% <0.00%> (+11.53%)	7.00% <0.00%> (+3.00%)
...edu/address/logic/commands/DescriptionCommand.java	25.00% <0.00%> (+25.00%)	2.00% <0.00%> (+2.00%)
...eedu/address/logic/commands/AddExpenseCommand.java	50.00% <0.00%> (+50.00%)	6.00% <0.00%> (+6.00%)
...dress/logic/parser/DeleteExpenseCommandParser.java	50.00% <0.00%> (+50.00%)	2.00% <0.00%> (+2.00%)
...c/main/java/seedu/address/logic/parser/Prefix.java	58.33% <0.00%> (+58.33%)	5.00% <0.00%> (+5.00%)
.../seedu/address/logic/parser/ViewCommandParser.java	66.66% <0.00%> (+66.66%)	2.00% <0.00%> (+2.00%)
... and 9 more

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