Add tests in ParserUtilTest

[matthewyip1511]

Unlike the what the name of the branch says, this PR only adds tests for ParserUtilTest 🙂 the tests for CommandUtilTest will be added in a separate PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	89.06% <100.00%> (+26.15%)	19.00 <0.00> (+4.00)