Problem:
- API functions using `try_start` directly instead of `TRY_WRAP`, do not
surface the underlying error message, and instead show generic things
like "Failed to set buffer".
- Error handling code is duplicated in the API impl, instead of
delegating to the vim buffer/window handling logic.
Solution:
- Use `TRY_WRAP`.
## Problem
With 'winfixbuf' enabled, `nvim_win_set_buf` and `nvim_set_current_buf` fail
even if targeting the already-current buffer.
vim.wo.winfixbuf = true
vim.api.nvim_win_set_buf(0, 0)
vim.api.nvim_set_current_buf(0)
Solution:
Check for this condition.
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.
