Refactor "server" tests in /tests/kola

[kelvinfan001]

Kolet can now easily start a HTTP server with the httpd command. This is especially useful for Zincati tests where we would like to have a mock Cincinnati server and mock OSTree repo server.

An Ignition file has been added in the tests/ directory with instructions on how to make use of it to conveniently create new integration tests.

Example usage can be found at e.g. https://github.com/coreos/zincati/blob/0e7aff5a26a7198dcd7bdf86a83b8fdd89c9488e/tests/kola/server/test-marker-file-strategy.sh

Actual test files under tests/server/ are quite long because they still need to create a mock Cincinnati graph to serve. Possibly create a test library to house functions to help create mock Cincinnati graphs to make the code in these test files less repetitive.