Test for issue #20

[Homescreen] Write test to access and exit edit mode
updated with assertRaises
updated according to code review
replaced assertRaises with assertEqual
improved _delete_app_locator
Updated TODO message
Merge remote-tracking branch 'mozilla/master' into issue_20
Merge remote-tracking branch 'mozilla/master' into issue_20
Redesigned asserts due to zac's comments
Merge pull request #411 from AlinT/issue_20

Test for issue #20