tests : add script to benchmark whisper.cpp on LibriSpeech corpus (#2999)

* tests : add script to benchmark whisper.cpp on LibriSpeech corpus

LibriSpeech is a widely-used benchmark dataset for training and
testing speech recognition models.

This adds a set of scripts to measure the recognition accuracy of
whisper.cpp models, following the common benchmark standards.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

* Document how to prepare `whisper-cli` and model files

Feedback from Daniel Bevenius.

This adds a short code example how to prepare the `whisper-cli`
command, to make the initial setup step a little bit clearer.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

* tests : Simplify how to set up Python environment

Based on a feedback from Georgi Gerganov.

Instead of setting up a virtual environment in Makefile, let users
set up the Python environment. This is better since users may have
their own preferred workflow/toolkit.

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

---------

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

tests/librispeech/requirements.txt

@@ -0,0 +1,6 @@
+# This is the minimal set of dependencies we need to compute
+# WER score. Read Section 3.2. of the original paper
+# (https://arxiv.org/abs/2212.04356) for more contexts.
+jiwer
+regex
+more-itertools