Implement non-fixture unit tests for STREAM

[sappenin]

The JS implementation of STREAM has some unit tests that we should ensure are covered in Quilt. Namely, these include the non-fixture tests found in https://github.com/interledgerjs/ilp-protocol-stream/blob/master/test/packet.test.ts as well as the STREAM tests here: https://github.com/interledgerjs/ilp-protocol-stream/blob/master/test/stream.test.ts

(Note, some or all of these cases might be covered or may not apply due to implementation differences, so it's worth doing some investigation first before blindly re-implementing the JS test cases)