Use custom cache location for integration tests.

The integration tests were being held hostage to whatever templates were
already in a user's cache. This made them unreliable, since some tests passed
because the template was in the cache. So the integration tests now override
the default cache location and delete the cache directory before running.

I have also rejigged the integration test build so that the test-tmpl package
is simply copied from src/integTest/resources rather than being installed by
the lazybones-templates build. In fact, we can probably remove that template
from the project.