Improvements to existing WebSMS tests

Improvements to existing WebSMS tests:
- test for mozMobileMessage.onsending event
- better error checking on user input for the destination number
- verify all fields in the found sms from mozMobileMessage.getMessage
- verify sms was sent from device before asking user to confirm the target
- give a few retries when sending SMS in case of bad network signal strength
- restructure some SmsTestCommon code