Auto merge of #124 - antrik:tests-receiver_set-etc, r=emilio
Auto merge of #124 - antrik:tests-receiver_set-etc, r=emilio

platform/test: More test cases for receiver sets etc.

This adds a bunch of new test cases improving coverage for
`OsIpcReceiverSet`; along with a pair of tests for sending empty
messages in general, which wasn't covered before at all.

servo/ipc-channel