efi_selftest: add a test for ResetSystem()

The unit test will reset the system by calling the ResetSystem() runtime
service before or after ExitBootServices() according to the users choice
by setting environment variable efi_selftest to:

* 'reset system' or
* 'reset system runtime'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>