Implement test cases for MedicalHistory suite of classes

[didymental]

EDIT: This PR is complete.

This PR resolves #266

[codecov-commenter]

Codecov Report

Merging #269 (8886e14) into master (dfe7068) will increase coverage by 3.62%.
The diff coverage is 73.07%.

@@             Coverage Diff              @@
##             master     #269      +/-   ##
============================================
+ Coverage     54.06%   57.68%   +3.62%     
- Complexity      572      607      +35     
============================================
  Files           118      118              
  Lines          2401     2408       +7     
  Branches        262      267       +5     
============================================
+ Hits           1298     1389      +91     
+ Misses         1021      933      -88     
- Partials         82       86       +4     

Impacted Files	Coverage Δ
...u/docit/logic/commands/AddMedicalEntryCommand.java	85.71% <0.00%> (+85.71%)	⬆️
...a/seedu/docit/storage/JsonAdaptedMedicalEntry.java	75.00% <ø> (+25.00%)	⬆️
...ocit/logic/commands/DeleteMedicalEntryCommand.java	88.00% <50.00%> (+88.00%)	⬆️
.../logic/parser/DeleteMedicalEntryCommandParser.java	92.85% <66.66%> (+92.85%)	⬆️
...java/seedu/docit/model/patient/MedicalHistory.java	74.50% <100.00%> (+11.74%)	⬆️
...c/main/java/seedu/docit/model/patient/Patient.java	91.66% <100.00%> (+14.58%)	⬆️
...in/java/seedu/docit/model/util/SampleDataUtil.java	29.16% <100.00%> (-12.22%)	⬇️
...a/seedu/docit/model/patient/UniquePatientList.java	88.88% <0.00%> (+2.22%)	⬆️
...main/java/seedu/docit/logic/parser/ParserUtil.java	71.83% <0.00%> (+2.81%)	⬆️
src/main/java/seedu/docit/model/Entry.java	82.60% <0.00%> (+4.34%)	⬆️
... and 7 more

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[huyuxin0429]

Pretty comprehensive tests of medical history class as well as its associated parser and commands. Good job!