Tidy up hacluster tests. In the process this fixes the error:
```
UnboundLocalError: local variable 'primary_status' referenced before assignment
```
This was caused by libjuju now returning an empty dict rather than
None when listing a subordinates units.
The hacluster tests assume that all the subordinates are hacluster.
However, with mysql8 there is a mysql-router subordinate as well, and it
doesn't support the clean-up action. This change ensures that the tests
checks that the subordinate IS hacluter prior to applying the
action/test.
The hacluster tests used to pause the hacluster subordinate but
then rely on the unintended side-effect of the prinicple going
into a blocked state to confirm the action was complete. This is
not what the old amulet tests used to do. Also, more importantly,
the side-effect has been fixed so the tests no longer worrk.
Aurelien Lourot
Liam Young
Alex Kavanagh
Xiyue Wang