ref 77594d55f709

Closes https://github.com/neovim/neovim/issues/10031

* tests: move os_kill to functional helpers

* tests: fix system_spec when run with clipboard manager

Replaces "xclip" with a dedicated helper program.

Fixes: https://github.com/neovim/neovim/issues/4900#issuecomment-501866842

Problem:    When starting with multiple tabs file messages is confusing.
Solution:   Set 'shortmess' when loading the other tabs. (Christian Brabandt)
https://github.com/vim/vim/commit/c75e81262347e47a69faabd72caf89fec3f06e8f

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

- Move os_name() up to "global helpers".
- Rename it to is_os().
- Make it depend on uname() instead of a running Nvim instance.

- Update the test to work with changes since it was originally written.
- Keep the test pending() because it still fails:

      Expected objects to be the same.
      Passed in:
      (table: 0x50ce9e38) {
        [1] = {
          [1] = 'notification'
          [2] = 'stderr'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } }
       *[2] = {
          [1] = 'notification'
          [2] = 'stdout'
         *[3] = {
            [1] = 0
           *[2] = {
             *[1] = '' } } }
        [3] = {
          [1] = 'notification'
          [2] = 'exit'
          [3] = {
            [1] = 0
            [2] = 143 } } }
      Expected:
      (table: 0x50ce9870) {
        [1] = {
          [1] = 'notification'
          [2] = 'stderr'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } }
       *[2] = {
          [1] = 'notification'
          [2] = 'stdout'
         *[3] = {
            [1] = 0
           *[2] = {
             *[1] = 'abcdef' } } }
        [3] = {
          [1] = 'notification'
          [2] = 'stdout'
          [3] = {
            [1] = 0
            [2] = {
              [1] = '' } } } }

Problem:  If a job exits while waiting on another job, the on_exit
          handler is queued but f_jobwait() skips it.
Solution: Always do process_wait(), so that handlers are run during
          f_jobwait().

fix #8302

Test case:
    $ BUSTED_ARGS="--repeat=2000 --no-keep-going" TEST_FILE=test/functional/core/job_spec.lua TEST_FILTER=waiting make functionaltest

Failure example (macOS CI):
    FAILED  test/functional/core/job_spec.lua: jobs jobwait will run callbacks while waiting
    test/functional/core/job_spec.lua:606: Expected objects to be the same.
    Passed in:
    (table: 0x1be77c80) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 3 } }
    Expected:
    (table: 0x1be77d10) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 4 } }
    stack traceback:
      test/functional/core/job_spec.lua:606: in function <test/functional/core/job_spec.lua:583

Test sometimes fails on macOS CI:

    FAILED  test/functional/core/job_spec.lua: jobs jobwait will run callbacks while waiting
    test/functional/core/job_spec.lua:606: Expected objects to be the same.
    Passed in:
    (table: 0x1be77c80) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 3 } }
    Expected:
    (table: 0x1be77d10) {
      [1] = 'notification'
      [2] = 'wait'
     *[3] = {
       *[1] = 4 } }
    stack traceback:
      test/functional/core/job_spec.lua:606: in function <test/functional/core/job_spec.lua:583

Change the test to check if all jobs are starting, not only exiting.