jobwait(): fix race if job exits before waiting on it

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

1 files changed, 0 insertions, 1 deletions

diff --git a/test/functional/core/job_spec.lua b/test/functional/core/job_spec.lua
index dec7c1422d..3a1be7bc52 100644
--- a/test/functional/core/job_spec.lua
+++ b/test/functional/core/job_spec.lua
@@ -607,7 +607,6 @@ describe('jobs', function()
       \  ])
       call rpcnotify(g:channel, 'wait', sort(g:jobs), sort(g:exits))
       ]])
-      assert:set_parameter('TableFormatLevel', 1000000)
       eq({'notification', 'wait',
         {{3,4,5,6}, {3,4,5,6}}}, next_msg())
     end)