test: adjust time-sensitive tests (#9220)

- window_split_tab_spec.lua: Put cursor at bottom of :terminal buffer so
  that it follows output.
- inccommand_spec.lua: Increase timeout to allow 2nd retry.
- 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.

test/functional/eval/timer_spec.lua

@@ -172,8 +172,8 @@ describe('timers', function()
         let g:val2 += 1
       endfunc
     ]])
-    command("call timer_start(50,  'MyHandler', {'repeat': 3})")
-    command("call timer_start(100, 'MyHandler2', {'repeat': 2})")
+    command("call timer_start(20, 'MyHandler',  {'repeat': 3})")
+    command("call timer_start(40, 'MyHandler2', {'repeat': 2})")
     run(nil, nil, nil, 300)
     eq(3,eval("g:val"))
     eq(2,eval("g:val2"))