These tests rely heavily on the pexpect library (inspired by TCL Expect); if you're editing or creating tests it is highly recommended you read up on the gotchas in here:
https://pexpect.readthedocs.io/en/stable/overview.html
If an expectation fails (or an unexpected timeout or EOF occurs) then pexpect will throw an exception which contains a printout of the internal state of the pexpect instance. This includes the buffer, before, and after buffers, which are essential for seeing what the child process's output was.
However, pexpect truncates two of these to 100 characters by default. To see more, set the undocumented str_last_chars attribute on the pexpect object to something larger, or to 0 for the full output.