Bug 838993 - Temporarily disable failing tests

These unit tests are currently failing and have been temporarily
disabled as per Bug 838993. They should be fixed and re-enabled
as soon as possible.

The following bugs track the process of re-enabling:

- Bug 838622 - [Dialer] USSD unit tests are failing
- Bug 840489 - [calendar] Fix failing unit tests
- Bug 840493 - [gallery] Fix failing unit tests
- Bug 840495 - [keyboard] Fix failing unit tests
- Bug 840497 - [sms] Fix failing unit tests
- Bug 840500 - [system] Fix failing unit tests