kselftest/alsa: pcm-test: Decrease stream duration from 4 to 2 seconds

kselftest/alsa: pcm-test: Decrease stream duration from 4 to 2 seconds

Currently test_pcm_time() streams audio on each PCM with each
configuration for 4 seconds. This time can add up, and with the current
45 second kselftest timeout, some machines like mt8192-asurada-spherion
can't even run to completion. Lower the duration to 2 seconds to cut the
test duration in half, without reducing the test coverage.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20230620220839.2215057-3-nfraprado@collabora.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>