… kong.conf.default

Since tests run with the conf loader in strict mode, any addition of directives
(or changes to directive enums) requires a change to kong/conf_loader.lua. In this
test, we verify that a PR including a change to this file also includes a change
to kong.conf.default.

When a change to kong.conf_loader does _not_ require a change in kong.conf.default
(for example, when performing an internal refactor), this test can be silenced by
adding the hashtag #noconfchange to the commit message.

This is still a approximation (an even stricter test would require kong.conf.default
to be changed in the same commit), but it should be sufficient to require all new
configuration entries added via pull requests to be documented.