tests: add decompress() tests for multiple frames and extra data
tests: add decompress() tests for multiple frames and extra data

Related to #59 and #181. The existing behavior isn't great. But we
should at least have tests to keep us honest if/when behavior changes.

indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard