(maint) Improve tests to check actual logging

The previous tests didn't actually make sure that the diff was logged.
This adds that coverage as well as checking that the log is at the
correct level.
(PUP-2033) Use loglevel for diff output

This brings the same changes made to the file type for PUP-1846 to the
augeas provider's diffing logic. If a loglevel is specified on the
resource then the diff will be logged at that level. If it is left off,
then the diff will be logged at the notice level.
Merge pull request #2469 from zaphod42/feature/master/pup-2033-augeas-show-diff

(PUP-2033) Use loglevel for diff output