(fix) Aider bench: logname fix; improve test calling instruction

[tobitege]

Short description of the problem this fixes or functionality that this introduces. This may be used for the CHANGELOG

Fixes potential logfile name if model has : character in it. Improve instruction on how to call the (optional) test case to prevent timeouts.

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Give a summary of what the PR does, explaining any non-trivial design decisions

• Any colon : in a model name, like nousresearch/hermes-3-llama-3.1-405b:extended prevents logging to file as there won't be a folder created.
• If the use of the unit test is enabled (env var option), the LLM's have all different ideas on how to execute that test, which often resulted in timeouts. The instruction to the LLM has been improved to use the same format as the bench would use itself,
  e.g. python -m unittest {instance.instance_name}_test.py

[xingyaoww]

LGTM!