test: Fix test expectation based on kernel config

Some test results are dependent on the kernel configuration option
CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE.

Check the kernel configuration file for its presence and expect appropriate
test results.

Function has_kernel_config is based on its xfstsests counterpart.

Signed-off-by: Pavel Reichl <preichl@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>