selftests/alsa: Add a few missing gitignore files

Compiled binary files should be added to .gitignore

'git status' complains:
Untracked files:
(use "git add <file>..." to include in what will be committed)
alsa/global-timer
alsa/utimer-test

Cc: Mark Brown <[e-mail]>
Cc: Jaroslav Kysela <[e-mail]>
Cc: Takashi Iwai <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Li Zhijian <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

tools/testing/selftests/alsa/.gitignore

···
+global-timer
 mixer-test
 pcm-test
 test-pcmtest-driver
+utimer-test