selftests/pstore: add pstore test script for pre-reboot

The pstore_tests script includes test cases which check pstore's
behavior before crash (and reboot).

The test cases are currently following.

- Check pstore backend is registered
- Check pstore console is registered
- Check /dev/pmsg0 exists
- Write unique string to /dev/pmsg0

The unique string written to /dev/pmsg includes UUID. The UUID is also
left in 'uuid' file in order to enable us to check if the pmsg keeps the
string correctly after reboot.

Example usage is following.

  # cd /path/to/selftests
  # make run_tests -C pstore (or just .pstore/pstore_tests)
  make: Entering directory '/path/to/selftests/pstore'
  === Pstore unit tests (pstore_tests) ===
  UUID=b49b02cf-b0c2-4309-be43-b08c3971e37f
  Checking pstore backend is registered ... ok
          backend=ramoops
          cmdline=console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootwait mem=768M ramoops.mem_address=0x30000000 ramoops.mem_size=0x10000
  Checking pstore console is registered ... ok
  Checking /dev/pmsg0 exists ... ok
  Writing unique string to /dev/pmsg0 ... ok
  selftests: pstore_tests [PASS]
  make: Leaving directory '/path/to/selftests/pstore'

We can also see test logs later.

  # cat pstore/logs/20151001-072718_b49b02cf-b0c2-4309-be43-b08c3971e37f/pstore_tests.log
  Thu Oct  1 07:27:18 UTC 2015
  === Pstore unit tests (pstore_tests) ===
  UUID=b49b02cf-b0c2-4309-be43-b08c3971e37f
  Checking pstore backend is registered ... ok
          backend=ramoops
          cmdline=console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootwait mem=768M ramoops.mem_address=0x30000000 ramoops.mem_size=0x10000
  Checking pstore console is registered ... ok
  Checking /dev/pmsg0 exists ... ok
  Writing unique string to /dev/pmsg0 ... ok

Signed-off-by: Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Salyzyn <salyzyn@android.com>
Cc: Seiji Aguchi <seiji.aguchi.tr@hitachi.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>