Now `:InspectTree` will show missing nodes as e.g. `(MISSING identifier)`
or `(MISSING ";")` rather than just `(identifier)` or `";"`. This is
doable because the `MISSING` keyword is now valid query syntax.
Co-authored-by: Christian Clason <c.clason@uni-graz.at>
Problem:
Tests have lots of exec_lua calls which input blocks of code
provided as unformatted strings.
Solution:
Teach exec_lua how to handle functions.
Problem:
1. When interacting with multiple :InspectTree and the source buffer
windows there is a high chance of errors due to the window ids not
being updated and validated.
2. Not all InspectTree windows were closed when the source buffer was
closed.
Solution:
1. Update InspectTree window id on `CursorMoved` event and validate
source buffer window id before trying to navigate to it.
2. Close all InspectTree windows
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.
