testbot: Fix the new errors and warnings detection in task.log files.
This includes:
- Using the task.log files of WineTest tasks as reference.
- Avoiding naming collisions in the above reference filenames.
- Avoiding over-deduplication of the task.log reference files.
- Grabbing only the task.log reference files relevant to the current task.
- Grabbing them early enough so they are available when analyzing the task.log file.