test: Add PStore command tests

Add PStore command to sandbox and sandbox64 defconfigs.
Add test checking:
- 'pstore display' of all records
- 'pstore display' only the 2nd dump record
- 'pstore save' of all records

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
[trini: Adjust to always load files from source directory]
Signed-off-by: Tom Rini <trini@konsulko.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>