efi_loader: selftest: enable APPEND_WRITE tests

Now that APPEND_WRITE is supported,
the result check for the only existing test case should be changed to
'todo' to 'error', while two more test cases are added.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>