SI-7600 [Avian] Skip tests r/stream_length and r/t4294

The issue is that Avian's GC is currently not precise enough to
determine the exact lifetime of each local/stack reference, and
therefore considers the this reference to be reachable in situations
where it could have been collected.

This can cause issues (as seen in run/stream_length and run/t4294:
`java.lang.OutOfMemoryError`) if code relies on the garbage collection
of these values to keep memory consumption constant.

This commit simply skips these two tests on Avian until the GC
implementation is fixed.
Merge pull request #2678 from soc/SI-7600

SI-7600 [Avian] Skip tests r/stream_length and r/t4294