efi_selftest: provide test for watchdog timer

The test verifies that resetting the watchdog timer ensures
that it is not called during the timeout period.

Testing that the watchdog timer actually executes a reset
would require a test outside the efi_selftest framework.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>