test/win: job_spec: increase jobwait() timeout

powershell is slow, and this timeout is "worst case" so it doesn't make
the tests more costly, only avoids a false-positive.

close #8958

test/functional/core/job_spec.lua

@@ -663,7 +663,7 @@ describe('jobs', function()
         call rpcnotify(g:channel, 'wait', jobwait([
         \  jobstart('exit 4'),
         \  jobstart((has('win32') ? 'Start-Sleep 10' : 'sleep 10').'; exit 5'),
-        \  ], has('win32') ? 3000 : 100))
+        \  ], has('win32') ? 6000 : 100))
         ]])
         eq({'notification', 'wait', {{4, -1}}}, next_msg())
       end)