tests: add fuzzing test for incremental block flushing
tests: add fuzzing test for incremental block flushing

If my reading of the zstd API documentation is correct, we may
be failing to handle ZSTD_compress_generic() returning != 0 when
passing ZSTD_e_flush. The added fuzzing test is an attempt to tease
that out.
indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard