PM / hibernate: Introduce test_resume mode for hibernation

test_resume mode is to verify if the snapshot data
written to swap device can be successfully restored
to memory. It is useful to ease the debugging process
on hibernation, since this mode can not only bypass
the BIOSes/bootloader, but also the system re-initialization.

To avoid the risk to break the filesystm on persistent storage,
this patch resumes the image with tasks frozen.

For example:
echo test_resume > /sys/power/disk
echo disk > /sys/power/state

[  187.306470] PM: Image saving progress:  70%
[  187.395298] PM: Image saving progress:  80%
[  187.476697] PM: Image saving progress:  90%
[  187.554641] PM: Image saving done.
[  187.558896] PM: Wrote 594600 kbytes in 0.90 seconds (660.66 MB/s)
[  187.566000] PM: S|
[  187.589742] PM: Basic memory bitmaps freed
[  187.594694] PM: Checking hibernation image
[  187.599865] PM: Image signature found, resuming
[  187.605209] PM: Loading hibernation image.
[  187.665753] PM: Basic memory bitmaps created
[  187.691397] PM: Using 3 thread(s) for decompression.
[  187.691397] PM: Loading and decompressing image data (148650 pages)...
[  187.889719] PM: Image loading progress:   0%
[  188.100452] PM: Image loading progress:  10%
[  188.244781] PM: Image loading progress:  20%
[  189.057305] PM: Image loading done.
[  189.068793] PM: Image successfully loaded

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>