efi: test/py: repair authenticated capsules tests

The UEFI console initialisation has been modified by commit 68edbed4
("efi_loader: initialize console size late"). A corresponding workaround is
now necessary for the automated tests, as added to some of the tests
already by commit e05bd68e ("test: work around for EFI terminal size
probing").

Add the same workaround to the UEFI authenticated capsules tests to repair
them.

This can be tested with sandbox_defconfig, sandbox64_defconfig or
sandbox_flattree_defconfig, plus CONFIG_EFI_CAPSULE_AUTHENTICATE=y.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>