tests: enforce all methods for config items

tests: enforce all methods for config items

This adds a test that checks LXC's configuration jump table whether all methods
for a given configuration item are implemented. If it is not, we'll error out.
This should provide additional safety since a) the API can now be sure that
dereferencing the pointer for a given method in the config struct is safe and
b) when users implement new configuration keys and forget to implement a
required method we'll see it right away.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>