Add more tests

[koh-jx]

Adding various tests

• Find task, undo for find
• Cases with multiple arguments
• Slash found in argument
• Added contacts in Test Builders and CommandTestUtil

Fix bugs:

• FindTaskCommand with no arguments not clearing filters as intended
• FindTaskCommand throwing NPE if no previous predicate exists (null)
• "Listed 1 tasks" not making grammatical sense: changed to "Listed 1 task(s)" (Same with persons)
• Undo message for FindTask and Find Commands not displaying the correct previous amounts of tasks/persons listed.

[codecov-commenter]

Codecov Report

Merging #189 (524c85c) into master (fc864e9) will increase coverage by 0.92%.
The diff coverage is 63.15%.

@@             Coverage Diff              @@
##             master     #189      +/-   ##
============================================
+ Coverage     60.82%   61.74%   +0.92%     
- Complexity      680      693      +13     
============================================
  Files           123      123              
  Lines          2757     2766       +9     
  Branches        264      266       +2     
============================================
+ Hits           1677     1708      +31     
+ Misses          985      964      -21     
+ Partials         95       94       -1     

Impacted Files	Coverage Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)
...u/address/logic/commands/task/FindTaskCommand.java	82.35% <60.00%> (+6.35%)	⬆️
...java/seedu/address/logic/commands/FindCommand.java	88.23% <75.00%> (+5.88%)	⬆️
...u/address/logic/commands/task/EditTaskCommand.java	97.97% <0.00%> (+1.01%)	⬆️
.../seedu/address/logic/parser/ArgumentTokenizer.java	93.10% <0.00%> (+3.44%)	⬆️
...va/seedu/address/logic/parser/CommandArgument.java	92.59% <0.00%> (+3.70%)	⬆️
...ress/model/task/TaskContainsKeywordsPredicate.java	85.71% <0.00%> (+7.14%)	⬆️
...in/java/seedu/address/logic/parser/ParserUtil.java	98.03% <0.00%> (+9.80%)	⬆️
...rc/main/java/seedu/address/model/task/Contact.java	66.66% <0.00%> (+16.66%)	⬆️
... and 2 more

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[mingyi456]

LGTM