Problem: Unpaired marks are shown with `filter.extmarks == true`, which
should only return visible highlights. Misleading `end_col`
included in `inspect_pos()` for unpaired mark; it is set to
`start_col + 1` which would be a visible highlight, which it is
not. Custom "is_here" filter used to get extmarks overlapping a
position.
Solution: Exclude unpaired highlight extmarks with `filter.extmarks == true`.
Set `end_col` to `start_col` for an unpaired mark. Supply
appropriate arguments to nvim_buf_get_extmarks() to return
overlapping extmarks; exclude marks whose end is at `{row, col}`
with `filter.extmarks == true`.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
