ALSA: pcmtest: Add support for pcm pausing

Add pause push/release support to the virtual PCM test driver. Add
'suspend' boolean field to the pcmtst_buf_iter structure, so we can
pause the timer without shutting it down. Update the trigger callback
handler correspondingly. Extract buffer initialization to the
'reset_buf_iterator' function since it is used in multiple places now.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230822150541.8450-1-ivan.orlov0322@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>