No global buildstamp for tests

This merge request removes the global build stamp for tests implemented in ./debian/rules. (It applies only when 'onlyrun' is not used.) The mechanism prevents needless re-runs, but also blocks runs that should be allowed, for example when only the test runner has changed. It a complete obstacle to runner development.

This merge request should not be accepted if any downstream consumers of Lintian rely on this mechanism to conserve resources.