The global variables are stored in $this, and in particular $db is used
through this mean. But $this->savdb is supposed to be the immutable
global state that is stored at the test class instantiation and restored
at the beginning of each test.
For $this->savdb, I don't think any consequences are created by this,
since the object is saved by each test anyway and the $db object is
mostly a query object within a transaction, but future change could use
this to inject a different stateful $db object to trace some behaviour
in the test for instance, so make sure the correct one is used.
