test: make Brotli 16GB test wait for backpressure

Wait for the Brotli decoder to fill its readable buffer before checking
that decompression stops at the high water mark. This avoids racing the
fixed timeout against libuv worker-pool scheduling.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr