selftests: alsa - move shared library configuration code to conf.c

The minimal alsa-lib configuration code is similar in both mixer
and pcm tests. Move this code to the shared conf.c source file.

Also, fix the build rules inspired by rseq tests. Build libatest.so
which is linked to the both test utilities dynamically.

Also, set the TEST_FILES variable for lib.mk.

Cc: linux-kselftest@vger.kernel.org
Cc: Shuah Khan <shuah@kernel.org>
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221129085306.2345763-1-perex@perex.cz


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