When you run a test in test mode there is a small window under the process that says "Test Results", that looks like this:
If your process hits a limit of 5,000 messages as described in the User Guide under Process testing, the logging will become truncated and you will see a message pop up like the one below.
When this message appears, the messages in the test results will no longer display the details about the test, but will instead just say that the shape is executing. In order to see the details of the shape execution, you will need to bring in less documents or limit what your process does in test mode. If the limits become an issue for testing, you can also deploy and run the process in a test environment in the Manage tab and analyze the process logs and test results in Manage.