Don't label merge train pipelines as merged result

What does this MR do and why?

I didn't consider that merge train pipelines always count as merged result pipelines in Add label to merged results pipelines (!132975 - merged), so both labels are currently applied. See !132975 (comment 1598942673) for details.

I've also added a test for this particular case.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

See original MR, but enable merge trains additionally

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