Add appointment parser tests

[licongshen12]

Fixes #176

[codecov]

Codecov Report

Merging #241 (1c8e4ff) into master (b84dad3) will increase coverage by 1.21%.
The diff coverage is 56.52%.

@@             Coverage Diff              @@
##             master     #241      +/-   ##
============================================
+ Coverage     73.63%   74.84%   +1.21%     
- Complexity      750      767      +17     
============================================
  Files           125      125              
  Lines          2446     2461      +15     
  Branches        239      240       +1     
============================================
+ Hits           1801     1842      +41     
+ Misses          537      511      -26     
  Partials        108      108              

Files	Coverage Δ
...seedu/address/logic/parser/ClearCommandParser.java	40.00% <100.00%> (ø)
.../seedu/address/logic/parser/ExitCommandParser.java	40.00% <100.00%> (ø)
.../seedu/address/logic/parser/HelpCommandParser.java	40.00% <100.00%> (ø)
.../seedu/address/logic/parser/ListCommandParser.java	40.00% <100.00%> (ø)
.../seedu/address/logic/parser/ModeCommandParser.java	40.00% <100.00%> (ø)
...r/appointmentparser/AppointmentsCommandParser.java	100.00% <100.00%> (+60.00%)	⬆️
...c/parser/appointmentparser/TodayCommandParser.java	100.00% <100.00%> (+60.00%)	⬆️
...arser/appointmentparser/UpcomingCommandParser.java	100.00% <100.00%> (+100.00%)	⬆️
...mands/appointmentcommands/AppointmentsCommand.java	71.42% <33.33%> (-28.58%)	⬇️
...gic/commands/appointmentcommands/TodayCommand.java	75.00% <33.33%> (-25.00%)	⬇️
... and 2 more

... and 5 files with indirect coverage changes

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[trgao]

LGTM