test/macOS: adjust time-sensitive tests

From test_timers.vim:
    Found errors in Test_paused():
    First run:
    function RunTheTest[35]..Test_paused line 20: Expected range 0 - 100, but got 123
    Second run:
    function RunTheTest[35]..Test_paused line 20: Expected range 0 - 100, but got 106

previously: #9220

- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b
  ref de13113dc1
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.

src/nvim/testdir/test_timers.vim

@@ -121,7 +121,7 @@ func Test_paused()
   let slept = WaitFor('g:val == 1')
   call assert_equal(1, g:val)
   if has('reltime')
-    call assert_inrange(0, 100, slept)
+    call assert_inrange(0, 140, slept)
   else
     call assert_inrange(0, 10, slept)
   endif