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 reverts commit c4c74c3883aa3122c0c877ca8dd7b26beb5cc4aa.

LibUV already gives an error for this, so this isn't needed.

Problem:    This doesn't give the right result: eval(string(v:true)). (Nikolay
            Pavlov)
Solution:   Make the string "v:true" instead of "true".
https://github.com/vim/vim/commit/f48aa160fdd7b8caa7678e1a2139244dd2bdc547

Problem:    The "trailing characters" error can be hard to understand.
Solution:   Add the trailing characters to the message.
https://github.com/vim/vim/commit/2d06bfde29bd3a62fc85823d2aa719ef943bd319

exepath and stdpath should respect shellslash and return path with
proper file separator.
Closes #13787

Problem:
Since right-click can now show a popup menu, we can provide messaging to
guide users who expect 'mouse' to be disabled by default. So 'mouse' can
now be enabled by default.

Solution:
Do it.
Closes #15521

These tests commonly fail in Debian with

    [  ERROR   ] test/functional/core/job_spec.lua @ 81: jobs append environment with pty #env
    test/functional/helpers.lua:198:
    ==============================================================================
    test/helpers.lua:73: Expected objects to be the same.
    Passed in:
    (table: 0x2aa4bcc1040) {
     *[1] = {
        [1] = 'notification'
        [2] = 'stdout'
       *[3] = {
          [1] = 0
         *[2] = {
            [1] = 'hello world abc' } } } }
    Expected:
    (table: 0x2aa4bcc0d70) {
     *[1] = {
        [1] = 'notification'
        [2] = 'stdout'
       *[3] = {
          [1] = 0
         *[2] = {
            [1] = 'hello world abc'
           *[2] = '' } } } }

Since the test is validating the "hello world abc" message, not the
newlines, accept either set of message sequences.

Problem:
- Since c57f6b28d71d #8519, sockets are created in ~/.local/… but XDG
  spec says: "XDG_RUNTIME_DIR: Must be on the local filesystem", which
  implies that XDG_STATE_DIR is potentially non-local.
- Not easy to inspect Nvim-created temp files (for debugging etc).

Solution:
- Store sockets in stdpath('run') ($XDG_RUNTIME_DIR).
- Establish "/tmp/nvim.user/" as the tempdir root shared by all Nvims.
- Make ok() actually useful.
- Introduce assert_nolog().

closes #3517
closes #17093