Add Tests for EditLoginCommand and Username

[peironggg]

Fixes #171
Fixes #172
Fixes #173

[codecov-io]

Codecov Report

Merging #170 into master will increase coverage by 5.38%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #170      +/-   ##
============================================
+ Coverage     45.84%   51.22%   +5.38%     
- Complexity      482      518      +36     
============================================
  Files           115      115              
  Lines          2360     2360              
  Branches        255      255              
============================================
+ Hits           1082     1209     +127     
+ Misses         1210     1062     -148     
- Partials         68       89      +21     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/seedu/address/model/login/Username.java	80.00% <ø> (+60.00%)	7.00 <0.00> (+6.00)
...rc/main/java/seedu/address/model/ModelManager.java	59.45% <0.00%> (+2.70%)	30.00% <0.00%> (+1.00%)
...main/java/seedu/address/commons/util/FileUtil.java	77.27% <0.00%> (+4.54%)	9.00% <0.00%> (+2.00%)
src/main/java/seedu/address/model/UserLogin.java	89.65% <0.00%> (+34.48%)	10.00% <0.00%> (+4.00%)
...ain/java/seedu/address/scraper/ScraperManager.java	37.20% <0.00%> (+37.20%)	14.00% <0.00%> (+14.00%)
...seedu/address/logic/commands/EditLoginCommand.java	92.30% <0.00%> (+92.30%)	7.00% <0.00%> (+7.00%)
...ss/commons/exceptions/OsNotSupportedException.java	100.00% <0.00%> (+100.00%)	1.00% <0.00%> (+1.00%)
...ss/commons/exceptions/ScraperParsingException.java	100.00% <0.00%> (+100.00%)	1.00% <0.00%> (+1.00%)

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[wireseo]

Nice test cases! LGTM