selftests/efivarfs: add check for disallowing file truncation

Now that the ability of arbitrary writes to set the inode size is
fixed, verify that a variable file accepts a truncation operation but
does not change the stat size because of it.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>