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.
Problem: Loading `vim.fs` via the `vim.loader` Lua package loader will
result in a stack overflow due to a cyclic dependency. This may happen
when the `vim.fs` module isn't byte-compiled, i.e. when `--luamod-dev`
is used (#27413).
Solution: `vim.loader` depends on `vim.fs`. Therefore `vim.fs` should
be loaded in advance.
Problem: cache paths are derived by replacing each reserved/filesystem-
path-sensitive char with a `%` char in the original path. With this
method, two different files at two different paths (each containing `%`
chars) can erroneously resolve to the very same cache path in certain
edge-cases.
Solution: derive cache paths by url-encoding the original (path) instead
using `vim.uri_encode()` with `"rfc2396"`. Increment `Loader.VERSION` to
denote this change.
