efi_selftest: prefix test functions with efi_st_

An upcoming patch set creates a global function flush(). To make debugging
easier we should not use the same name for a static function.

Rename static functions in the LoadImage() unit test adding an efi_st_
prefix.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>