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

runtime/doc/eval.txt

@@ -5411,7 +5411,8 @@ jobstop({id})						*jobstop()*
 		See |job-control|.
 
 jobwait({ids}[, {timeout}])				*jobwait()*
-		Wait for a set of jobs to complete.
+		Wait for a set of jobs and their |on_exit| handlers to
+		complete.
 
 		{ids} is a list of |job-id|s to wait for.
 		{timeout} is the maximum waiting time in milliseconds, -1
@@ -5427,10 +5428,10 @@ jobwait({ids}[, {timeout}])				*jobwait()*
 		Returns a list of len({ids}) integers, where each integer is
 		the wait-result of the corresponding job. Each wait-result is
 		one of the following:
-			* Exit-code, if the job exited
-			* -1 if the timeout was exceeded
-			* -2 if the job was interrupted
-			* -3 if the |job-id| is invalid
+			Exit-code, if the job exited
+			-1 if the timeout was exceeded
+			-2 if the job was interrupted (by |CTRL-C|)
+			-3 if the job-id is invalid
 
 join({list} [, {sep}])					*join()*
 		Join the items in {list} together into one String.

src/nvim/eval.c

@@ -12487,7 +12487,7 @@ static void f_jobwait(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     }
 
     // if the job already exited, but wasn't freed yet
-    if (jobs[i] == NULL || jobs[i]->stream.proc.status >= 0) {
+    if (jobs[i] == NULL) {
       continue;
     }
 

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)