Problem:
Tests that _intentionally_ fail certain conditions cause noise in
$NVIM_LOG_FILE:

    $NVIM_LOG_FILE: /home/runner/work/neovim/neovim/build/.nvimlog
    (last 100 lines)
    WRN 2023-01-16T18:26:27.673 T599.7799.0 unsubscribe:519: RPC: ch 1: tried to unsubscribe unknown event 'doesnotexist'
    WRN 2023-01-16T18:29:00.557 ?.11151    server_start:163: Failed to start server: no such file or directory: /X/X/X/...
    WRN 2023-01-16T18:33:07.269 127.0.0.1:12345 server_start:163: Failed to start server: address already in use: 127.0.0.1
    ...
    -- Output to stderr:
    module 'vim.shared' not found:
    	no field package.preload['vim.shared']
    	no file './vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/share/lua/5.1/vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/share/lua/5.1/vim/shared/init.lua'
    	no file '/home/runner/nvim-deps/usr/lib/lua/5.1/vim/shared.lua'
    	no file '/home/runner/nvim-deps/usr/lib/lua/5.1/vim/shared/init.lua'
    	no file './vim/shared.so'
    	...
    E970: Failed to initialize builtin lua modules

Solution:
- Log to a private $NVIM_LOG_FILE in tests that intentionally fail and
  cause ERR log messages.
- Assert that the expected messages are actually logged.

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())`.

The environment variable CIRRUS_CI is manually passed to RunTests.cmake
as it doesn't get passed when using cmake script mode.

dispatch.sr.ht is being deprecated, meaning that using sourcehut CI
won't be possible (see https://github.com/neovim/neovim/issues/19609).
Since Github Actions doesn't provide any BSD runners an external service
is required and Cirrus CI seems like a good replacement for sourcehut.

Initially experimented with using FreeBSD and OpenBSD virtual machines
in GitHub Actions, but Cirrus has been a much better fit with better
performance, logs and overall experience.

Failing tests are automatically skipped on FreeBSD regardless if it's on
CI or not. Ideally these tests should only be skipped in CI with the
help of `isCI` helper function. Unfortunately, the tests don't recognize
the environment variable CIRRUS_CI even if it's set manually. This
workaround is good enough for the time being, but we might want to only
skip tests when using the CI (or even better, fix the failing tests).

Closes: https://github.com/neovim/neovim/issues/19609

This is already skipped in all CI environments, so it should also be
skipped in environments that don't like fragile tests.  Since there's no
convenient way to express these concisely, add the explicit fragile
skip.

ref #15440