tests: add test for dvc.api.open with remote

[skshetry]

Covers #7341.

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[skshetry]

@efiop, this still fails on:

dvc/dvc/fs/dvc.py

Line 172 in b6fa117

outs = list(self.repo.index.tree.iteritems(key))

(you can try placing breakpoint here on this test)

[efiop]

On closer inspection, unlike before data tree introduction, this rises a somewhat unnoticed before problem of remote argument and metadata that the repo structure is getting built from. Plus every output might use its own remote, so remote becomes even more questionable. What we might be looking for here is some kind of config (mapping?) adjustment. Some use cases that come to mind:

1. change default core.remote to something else (dvc config core.remote myremote)
2. adjust particular remote configuration (e.g. add some keys or something) (dvc remote modify myremote key mykey)
3. adjust remotes for particular outputs (currently no CLI command, so can only be modified by opening dvc.yaml/data.dvc and editing by hand)

For the most part, it seems like current remote= argument is mostly used to emulate 1).

Overall this seems to be the same problem as providing config through cli/api, e.g. #2466

1 and 2 can be solved by supporting config= in API (the values will be merged with repo config and data tree will be built using it, so you won't need to provide remote= to open at all, which is nice), but option 3 seems to require something more advanced (data tree editing?).