**Problem:**
The brackets in the RFC2732 regular expression are currently unescaped,
causing them to be misinterpreted as special characters denoting
character groups rather than as literal characters.

**Solution:**
Escape the brackets.
Fix #31270

- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
  pervasive convention, not `\` slashes.

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.

Work on https://github.com/neovim/neovim/issues/27004.

Problem: Some LSP servers return `textDocument/documentLink` responses
         containing file URIs with line/column numbers in the fragment.
         `vim.uri_to_fname` returns invalid file names for these URIs.

Solution: Remove the URI fragment from file URIs.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

This is essentially a convenience wrapper around the `pending()`
function, similar to `skip_fragile()` but more general-purpose.

Also remove `pending_win32` function as it can be replaced by
`skip(iswin())`.

Co-authored-by: Raphael <glephunter@gmail.com>
Co-authored-by: smjonas <jonas.strittmatter@gmx.de>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>