Device-specific asserts

[xpconanfan]

Similar to device.log, which prepends device debug tag to the log msg, it would be nice to have a set of asserts functions specific to a device.

This way we could simply do

device.assert_true(xxx, msg)

instead of having to do string formatting every time:

asserts.assert_true(xxx, '%s msg` % device.debug_tag)