tests: don't build audio tests when no audio drivers are enabled

When there are no audio drivers configure fails with "ERROR: Command
cannot have '@INPUT@', since no input files were specified".

Fixes: 3220b38a8d ("tests: start manual audio backend test")
Signed-off-by: Anton Kuchin <antonkuchin@nebius.com>
Message-ID: <DB8P190MB07142E1BA8DEEA8B2E41D5B2DB57A@DB8P190MB0714.EURP190.PROD.OUTLOOK.COM>
[ Marc-André - use empty modinfo stub ]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
2 files changed