Three of the shapes worth covering produce frames of a few hundred kilobytes,
which is too much to commit for what they prove. The input is built from a
seeded generator and handed to the zstd CLI instead, so the frame is libzstd's
own output and the expectation is the exact bytes that went in.
The pool size decides which sequence tables libzstd picks. Two hundred tokens
gets RLE and repeat tables for the literal lengths, two thousand gets an RLE
table for the match lengths, and both put more than 32,512 sequences in one
block, which is the only thing that writes the three-byte sequence count.
