Refactoring OperatorViewDetachedFromWindowFirst

1. Renaming to OnSubscribeViewDetachedFromWindowFirst.java
2. Adding tests to assert the desired behaviour
3. Refactoring to avoid the "instantiate and forget" approach
within OnSubscribe.call(..)

Signed-off-by: David Marques <dpsmarques@gmail.com>
Use `org.junit.Assert`. `junit.*` is all legacy.