Rather than having every "enqueue"-type function have an output pointer
specifically for an output event, just provide an `olCreateEvent`
entrypoint which pushes an event to the queue.
For example, replace:
```cpp
olMemcpy(Queue, ..., EventOut);
```
with
```cpp
olMemcpy(Queue, ...);
olCreateEvent(Queue, EventOut);
```
Add `OffloadDeviceTest::getPlatformBackend()` and use it to skip event
tests which currently fail on AMDGPU due to:
```
OL_ERRC_UNIMPLEMENTED: synchronize event not implemented
```
Add info queries for queues and events.
`olGetQueueInfo` only supports getting the associated device. We were
already tracking this so we can implement this for free. We will likely
add other queries to it in the future (whether the queue is empty, what
flags it was created with, etc)
`olGetEventInfo` only supports getting the associated queue. This is
another thing we were already storing in the handle. We'll be able to
add other queries in future (the event type, status, etc)
A couple of liboffload entry points were missed out from the tests, and
unsurprisingly a crash in one of them made it in. Add the tests and fix
the unchecked error in `olDestroyEvent`.
