ALSA: kunit: use const qualifier for immutable data

Some data for testing is immutable. In the case, the const qualifier is
available for any loader to place it to read-only segment.

Fixes: 3e39acf5 ("ALSA: core: Add sound core KUnit test")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Message-ID: <20240425233653.218434-1-o-takashi@sakamocchi.jp>