Fix instability detection tagging

[kivistikliis]

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Purpose of this PR

• Removes adding a redundant unknown tag, if any pattern from any log is matched (except for successful-retry)
• Overwrites matched pattern conclusion (which is usually failure) to be success for all successful-retries
• Enforces having either successful-retry or instability tag, never both (having both would complicate distinguishing between successful retries, failed instabilities, and failed retries which are not instabilities. Refer to docs for more elaborate explanation)
• Some other smaller rewrites

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Testing status

Tested locally on several execution logs, as well as the new ones from this and this job, which brought these issues to attention.

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Comments to reviewers

This is properly tested via Yamato, but as instabilities occur by random, the best way is to investigate the output after nightlies.

[kivistikliis]

Merging without explicit approval, as this was discussed and approved internally.