TST: Add tests mapping out the rules for metadata in promotion

Note that these rules do not actually make much sense often.
They depend on a lot of suble branches, which probably grew over
time without much regard for metadata.

The test is ugly, but maps them out to detect changes in behaviour.

At some point, we should probably define how metadata and promotion
works.  Such as always merging dicts, always drop if the original
dtype is lost?
That assumes that metadata can be a useful concept which makes sense
to "inherit" during promotion.
Its also plausible that we should just always strip metadata during
promotion.

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