tldr; Placeholder until after https://jira.mongodb.org/browse/INTPYTHON-492 is resolved.

This directory had previously held copies of test_pregel.py and test_pregel_async.py,
as well as associated fixtures and utilities.
[See here](https://github.com/langchain-ai/langgraph/blob/main/libs/langgraph/tests)
What was here was only temporary.
The files were copied from the langgraph library, not langgraph-checkpoint-X.
While not in the checkpoint lib itself, they provided extensive testing of the checkpointers
that were crucial to testing edge cases.

As LangGraph development continued our copies have began to fail.
We will determine how to proceed with maintenance in conversation with the
Langchain-AI / LangGraph team.
