The `cluster diff` command is now tested by doing add/alter/remove
operations on servers. As the same methods are used for all objects, they
should detect all sorts of modifications between two Maxscales.
The tests are now sorted by command type. This allows for more consistent
test creation as tests can be done per source code file instead of per
object type.
Added helper functions for common testing operations. The two main ones
are doCommand, which executes a MaxCtrl command and returns a result, and
verifyCommand, that executes a command and verifies the result via the
REST API.
