selftests: add tests for efivarfs

This change adds a few initial efivarfs tests to the
tools/testing/selftests directory.

The open-unlink test is based on code from Lingzhu Xiang.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Cc: Matt Fleming <matt.fleming@intel.com>
Cc: Lingzhu Xiang <lxiang@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>