Make bandit specs optional

As described in joakimkarlsson/bandit#43, introduces options for building and running the tests.

A parent project can then override the options to prevent the tests from building and running.

In addition, the snowhouse-tests custom target is removed because it never runs (joakimkarlsson/bandit#42).

Depends on joakimkarlsson/snowhouse/pull/4 so if this gets merged, build and release snowhouse first.