Add MIRROR Jinja2 variable in rose-stem and associated test

Some of our scripts fail in Rose as they access the shared repository and do not have permission when not run interactively. The best solution we've come up with is to redirect these to run from the mirror. This change adds a Jinja2 variable SOURCE_FOO_MIRROR set by the "rose stem" command which is set to the equivalent internal mirror location of the branch.

This is required in site-default rose before we move to the shared repository.

I've also updated the test battery so that the rose-stem tests do something which is slightly closer to how the system will actually be used.
Is this better?

```python
basedir = self._ascertain_project(os.getcwd())[1]
```