test: fix zlib-brotli output assumptions

On different architectures, it's possible for the compression algorithm
to produce slightly different outputs. So, don't assume we will
always get the same compressed output on all architectures. Instead,
verify that the decompressing pre-compressed string functions
correctly.

Fixes: https://github.com/nodejs/node/issues/25568

PR-URL: https://github.com/nodejs/node/pull/25697


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>