tests/test-gendoc-fr.t

rust: mostly avoid streaming zstd decompression

Streaming ZStd decompression seems slightly slower, and
the API we use makes it very inconvenient to re-use the
decompression context.

Instead of using that, use the buffer-backed version,
because we can give a reasonable-ish size estimate.

#require docutils gettext

  $ $TESTDIR/check-gendoc fr
  checking for parse errors