Add tests

[jzwoo]

Additional tests have been added to:

OneWordCommandParserTest
FindModCommandParserTest
FindCommandParserTest
AssignCommandParserTest
AddressBookParserTest
AddModCommandParserTest
FindModCommandTest
FindCommandTest

Fixes #238

[codecov-io]

Codecov Report

Merging #242 (df50736) into master (4fbcfac) will increase coverage by 0.08%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #242      +/-   ##
============================================
+ Coverage     72.87%   72.96%   +0.08%     
- Complexity      762      776      +14     
============================================
  Files           109      109              
  Lines          2205     2212       +7     
  Branches        275      275              
============================================
+ Hits           1607     1614       +7     
+ Misses          506      502       -4     
- Partials         92       96       +4     

Impacted Files	Coverage Δ	Complexity Δ
...va/seedu/address/logic/commands/CclearCommand.java	85.71% <0.00%> (-14.29%)	4.00 <1.00> (+1.00)	⬇️
...ava/seedu/address/logic/commands/ClistCommand.java	80.00% <0.00%> (-20.00%)	3.00 <1.00> (+1.00)	⬇️
...java/seedu/address/logic/commands/ExitCommand.java	66.66% <0.00%> (-33.34%)	3.00 <1.00> (+1.00)	⬇️
...java/seedu/address/logic/commands/HelpCommand.java	66.66% <0.00%> (-33.34%)	3.00 <1.00> (+1.00)	⬇️
...va/seedu/address/logic/commands/MclearCommand.java	85.71% <0.00%> (-14.29%)	4.00 <1.00> (+1.00)	⬇️
...ava/seedu/address/logic/commands/MlistCommand.java	80.00% <0.00%> (-20.00%)	3.00 <1.00> (+1.00)	⬇️
...ava/seedu/address/logic/commands/ResetCommand.java	80.00% <0.00%> (-20.00%)	3.00 <1.00> (+1.00)	⬇️
.../seedu/address/logic/parser/FindCommandParser.java	83.78% <0.00%> (+2.70%)	19.00% <0.00%> (+4.00%)
...edu/address/logic/parser/OneWordCommandParser.java	92.00% <0.00%> (+4.00%)	22.00% <0.00%> (+1.00%)
...edu/address/logic/parser/FindModCommandParser.java	88.88% <0.00%> (+14.81%)	12.00% <0.00%> (+2.00%)

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[justintzuriel]

LGTM