Enforcing pylint standards and default for our test code seems
counter-productive. Since most of the newly added code are tests or is
test-related, encountering these checks rarely make us refactor the code
in other ways and we just disable these checks individually. Code that
is too complex or convoluted will be pointed out in reviews anyways.
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.
