test/py: efi_secboot: fix test case 1g of test_authvar

In the test case (1g) of test_authvar, "db" is mistakenly used,
and it ends up being the exact same as (1f).
So correct it as "dbx" test case.

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