test/py: efi_secboot: split "signed image" test case-1 into two cases

Split the existing test case-1 into case1 and a new case-2:
case-1 for non-SecureBoot mode; case-2 for SecureBoot mode.

In addition, one corner case is added to case-2; a image is signed
but a corresponding certificate is not yet installed in "db."

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>