Debian/Ubuntu have *helpfully* removed gdbm from the base python
package and put it into another package, which is not dragged in by
installing python. Testr doesn't function without this.

We should ensure this always gets installed.

Depends-On: If48a8444b02ee1e105bc1d9ce78a0489ea0c405b

Change-Id: I85a0ffe5ee384e055e78fd3164c27d42a86bc39d