selftests: ALSA: fix memory leak in utimer test

Free the malloc'd buffer in TEST_F(timer_f, utimer) to prevent
memory leak.

Fixes: 1026392d ("selftests: ALSA: Cover userspace-driven timers with test")
Reported-by: Jun Zhan <zhanjun@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Link: https://patch.msgid.link/DE4D931FCF54F3DB+20250731100222.65748-1-wangyuli@uniontech.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>