newsyslog: fix compression arglist construction

Reuse of the index variable in two nested loops resulted in only the first
argument in the list being used (fine for gzip, not fine for zstd). Also
add tests for xz and zstd, and fix the COMPRESS_SUFFIX_MAXLEN macro.

Submitted by: dnelson_1901_yahoo.com
Differential Revision: https://reviews.freebsd.org/D16509