test: relax chunk count expectations

In parallel/test-fs-read-stream-concurrent-reads.js the number
of data chunks used is being tested when few concurrent reads
are performed. The number of chunks can fluctuate based on the
number of concurrent reads as well as the data that was read in
one shot. Accommodate these variations in the test.

Fixes: https://github.com/nodejs/node/issues/22339

PR-URL: https://github.com/nodejs/node/pull/25415


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