Rename files to match the new content

We don't plan to add more predicates for now, so we rename
`_predicates.py` to `_with_previous.py` and
`test_predicates_only_if_previous.py` to `test_with_previous.py`.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/channels/experimental/__init__.py

@@ -10,8 +10,8 @@
 """
 
 from ._pipe import Pipe
-from ._predicates import WithPrevious
 from ._relay_sender import RelaySender
+from ._with_previous import WithPrevious
 
 __all__ = [
     "WithPrevious",