[JENKINS-17575] Baseline testing of behavior without folders.
(cherry picked from commit dcff6e41302093b6ba90da0aa6e9b09a5cfe2b86)
[FIXED JENKINS-17575] Redirecting properly after deleting jobs in folders.
Expanded MockFolder to be a ViewGroup so that it is possible to test views inside folders,
and generally fixed it to service Stapler hierarchies properly so that WebClient-based tests can work.
(cherry picked from commit 1205e9f6c492c53983a7f02926a53739b50b92fd)