Test for the self-issued predicate

PR input-output-hk/ouroboros-network#2930 added a function for the node to use to identify headers of self-issued blocks (for use in a metric), but we don't have any tests for that function. This Issue is to add a test, for that, probably in the ThreadNet tests.

I set medium priority since if this predicate is wrong, it will lead to non-sense numbers in a metric that node operators care about.