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/Makefile

@@ -0,0 +1,15 @@
+TAR_URL = https://www.openslr.org/resources/12/test-clean.tar.gz
+
+all: eval
+
+eval:
+	$(MAKE) -f eval.mk
+
+clean:
+	$(MAKE) -f eval.mk clean
+
+get-audio:
+	wget -c $(TAR_URL)
+	tar -xf test-clean.tar.gz
+
+.PHONY: all eval clean setup-venv clean-venv get-audio