When `test_autoretrain_dict` was originally written, the default
`sstable_compression_user_table_options` was `LZ4Compressor`. The
test assumed (correctly) that initially the compression doesn't use
a trained dictionary, and later in the test scenario, it changed
the algorithm to one with a dictionary.
However, the default `sstable_compression_user_table_options` is now
`LZ4WithDictsCompressor`, so the old assumption is no longer correct.
As a result, the assertion that data is initially not compressed well
may or may not fail depending on dictionary training timing.
To fix this, this commit explicitly sets `ZstdCompressor`
as the initial `sstable_compression_user_table_options`, ensuring that
the assumption that initial compression is without a dictionary
is always met.
Note: `ZstdCompressor` differs from the former default `LZ4Compressor`.
However, it's a better choice — the test aims to show the benefit of
using a dictionary, not the benefit of Zstd over LZ4 (and the test uses
ZstdWithDictsCompressor as the algorithm with the dictionary).
Fixes: scylladb/scylladb#28204
(cherry picked from commit 9ffa62a986)
c46ae2c2ab