utest: Fix test case SignatureTest.TestSignatureVerify()

The unit test case SignatureTest.TestSignatureVerify() passes
with default config.
But when `verify-full-signature` is enabled, the test case fails.

resolved: openbmc/phosphor-bmc-code-mgmt#9

Tested: Unit Test successfully when `verify-full-signature` is
enabled.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ia600690ea26ca3559f1e9f2049283880ddf95a4d
1 file changed