mgr: generic self test command

mgr: generic self test command

Avoid need for each module to expose a self-test
command: they can just implement the method,
and then get it called via the selftest module.

As well as fewer LOC, this means that the self
test commands are not cluttering the interface
for end users, as they've invisible until
the selftest module is loaded.

Signed-off-by: John Spray <john.spray@redhat.com>