Add tests for Shift model classes

[WangZijun97]

Tests added for the following classes:

• Shift
• ShiftDay
• ShiftTime
• RoleRequirements
• Role
• UniqueShiftList
• ShiftDayOrTimeContainsKeywordPredicate

which should cover testing for seedu.address.model.shift sufficiently.

Relevant to #43 and builds upon PR #49

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@eba28f1). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #50   +/-   ##
=========================================
  Coverage          ?   65.17%           
  Complexity        ?      509           
=========================================
  Files             ?       92           
  Lines             ?     1720           
  Branches          ?      195           
=========================================
  Hits              ?     1121           
  Misses            ?      554           
  Partials          ?       45           

Impacted Files	Coverage Δ	Complexity Δ
...ava/seedu/address/model/shift/RoleRequirement.java	78.57% <ø> (ø)	15.00 <0.00> (?)
src/main/java/seedu/address/model/AddressBook.java	86.36% <100.00%> (ø)	19.00 <5.00> (?)
...rc/main/java/seedu/address/model/ModelManager.java	92.06% <100.00%> (ø)	28.00 <0.00> (?)
...shift/ShiftDayOrTimeContainsKeywordsPredicate.java	100.00% <100.00%> (ø)	9.00 <9.00> (?)

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