Support stream testing without expecting a decompressed stream

In some cases like [sqlite-compressions](https://github.com/nyurik/sqlite-compressions), a user may need to validate the integrity of a compressed value, but not care to get the decoded value, i.e. `brotli_test(brotli("foo bar"))` should return true. Currently, there doesn't seem to be any efficient way to do a `decode to /dev/null` (i.e. without actually copying the result to the output).