efi: Split out test init/uninit into functions

The functions in bootefi are very long because they mix high-level code
and control with the low-level implementation. To help with this, create
functions which handle preparing for running the test and cleaning up
afterwards.

Also shorten the awfully long variable names here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>