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.
Move test cases that are more about treesitter query API rather than
parser API or LanguageTree out of "treesitter/parser_spec", and collect
them in another test suite "treesitter/query_spec".