Move API tests to /integration/

closes #1396
- moved core/test/unit/api* to core/test/integration/api/
- moved core/test/integration/model* to core/test/integration/model/
- moved core/test/unit/utils to core/test/utils
- moved core/test/unit/fixtures to core/test/utils/fixtures/
- changed gruntfile.js to execute api tests with target 'integration'