Add additional testing of FSST encoding

[westonpace]

Currently the FSST encoding is only applied when the data is larger than 4MiB. This is an artificial limitation because I was seeing bugs when encoding smaller strings. In addition, FSST is fairly complex, and we need to be highly confident that it will not write corrupt data (writer corruption is difficult to recover from since the data is usually lost).

Let's enhance our confidence in FSST with some additional testing and make sure any threshold in FSST application is meaningful and well understood.