Support stream testing without expecting a decompressed stream

[nyurik]

In some cases like 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).