Cache path should be injected into template engine

Currently cache path is hardcoded to be boardroot/cache in the template engine. This creates a number of issues:

1. The same path is used by tests and when running the code.
2. Tests retrieve the path from the template engine instead of feeding the path they want to the template engine.
3. Template tests refuse to run if the source tree is read-only.

The first step is making the cache path a parameter to the template engine. The tests then can specify a path, which later may be made configurable.