selftests: firmware: Add compressed firmware tests

This patch adds the test cases for checking compressed firmware load.
Two more cases are added to fw_filesystem.sh:
- Both a plain file and an xz file are present, and load the former
- Only an xz file is present, and load without '.xz' suffix

The tests are enabled only when CONFIG_FW_LOADER_COMPRESS is enabled
and xz program is installed.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>