selftests: firmware: Simplify test patterns

The test patterns are almost same in three sequential tests.
Make the unified helper function for improving the readability.

Link: https://lore.kernel.org/all/20210127154939.13288-1-tiwai@suse.de/


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20220421152908.4718-5-tiwai@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>