Problem:
- When multigrid is enabled, screenchar()/screenstring() functions return wrong
results. See https://github.com/neovide/neovide/issues/2569
- `screenstring()` executed via RPC in child Nvim process, doesn't recognize
floating windows.
Solution:
In ui_comp_get_grid_at_coord(), also iterate window grids.
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.
