Move all test cases from tests.sh to tests_ksr.py. The only test that
is not moved is the check that key id's match expected keys. The
shell-based system test checks two earlier set environment variables
against each other that has become redundant in the pytest variant,
because we now check the signed key response against a list of keys
and for each key we take into account the timing metadata. So we
already ensure that each published key is in the correct key bundle.